Method of mode coupling detection and damping and usage for electrostatic mems mirrors

ABSTRACT

A scanning system includes a microelectromechanical system (MEMS) scanning structure configured with a desired rotational mode of movement based on a driving signal; a plurality of comb-drives configured to drive the MEMS scanning structure according to the desired rotational mode of movement based on the driving signal, each comb-drive including a rotor comb electrode and a stator comb electrode that form a capacitive element that has a capacitance that depends on the deflection angle of the MEMS scanning structure; a driver configured to generate the at least one driving signal; a sensing circuit selectively coupled to at least a subset of the plurality of comb-drives for receiving sensing signals therefrom, wherein each sensing signal is representative of the capacitance of a corresponding comb-drive; and a processing circuit configured to determine a scanning direction of the MEMS scanning structure in the desired rotational mode of movement based on the sensing signals.

BACKGROUND

Light Detection and Ranging (LIDAR), is a remote sensing method thatuses light in the form of a pulsed laser to measure ranges (variabledistances) to one or more objects in a field of view. In particular, amicroelectromechanical system (MEMS) mirror is used to scan light acrossthe field of view. Arrays of photodetectors receive reflections fromobjects illuminated by the light, and the time it takes for thereflections to arrive at various sensors in the photodetector array isdetermined. This is also referred to as measuring time-of-flight (ToF).LIDAR systems form depth measurements and make distance measurements bymapping the distance to objects based on the time-of-flightcomputations. Thus, the time-of-flight computations can create distanceand depth maps, which may be used to generate images.

In order to improve the MEMS mirror scanning performance by achievinghigh scanning frequencies, it is a goal to design the MEMS mirror to belightweight. The mirror's moment of inertia can be reduced by choosing alow thickness of the mirror plate. However, lighter mirror plates with alow thickness lead to increased dynamic deformation at the mirror for afixed frequency or trajectory (equals to a set of frequencies). Thisdeformation causes distortion in transmitted and/or received lightbeams. For this reason, the mirror plate is supported by reinforcementstructures, which aid in suppressing dynamic deformation. However, theinclusion of the reinforcement structures leads to a coupling of thedesired rotational mode Rx about the mirror's scanning axis to anunwanted in-plane translational mode Ty. The operation point where theTy resonance occurs varies for each device and therefore is hard topredict or to avoid by design. Another unwanted parasitic mode, a yawmode Rz, also depends on the ambient temperature. For example, as theYoung's modulus is in general temperature dependent, parasitic modessuch as the yaw mode Rz may shift in resonance frequency. In addition,such parasitic mode can be excited the high order harmonic components ofthe actuation signal or main oscillation motion, which coincide otherrigid body modes. The detection and avoidance of such operation pointsexhibiting strong coupling to unwanted parasitic modes can be criticalto ensure the proper operation of the MEMS mirror. To ensure highposition accuracy and respective image resolution while shooting a laserpulse, a precise position estimation of the mirror is required.

Therefore, an improved control structure for detecting, measuring,and/or counteracting parasitic modes may be desirable.

SUMMARY

One or more embodiments provide a scanning system, including:

microelectromechanical system (MEMS) scanning structure configured torotate about an axis with a desired rotational mode of movement based onat least one driving signal;

a plurality of comb-drives configured to drive the MEMS scanningstructure about the axis according to the desired rotational mode ofmovement based on the at least one driving signal, wherein eachcomb-drive includes a rotor comb electrode and a stator comb electrodethat form a capacitive element that has a capacitance that depends onthe deflection angle of the MEMS scanning structure;

a driver configured to generate the at least one driving signal;

a sensing circuit selectively coupled to at least a subset of theplurality of comb-drives for receiving sensing signals therefrom,wherein each sensing signal is representative of the capacitance of acorresponding comb-drive; and

a processing circuit configured to determine a scanning direction of theMEMS scanning structure in the desired rotational mode of movement basedon the sensing signals.

One or more embodiments provide a scanning system, including:

microelectromechanical system (MEMS) scanning structure configured torotate about an axis with a desired rotational mode of movement based onat least one driving signal;

a plurality of comb-drives configured to drive the MEMS scanningstructure about the axis according to the desired rotational mode ofmovement based on the at least one driving signal, wherein eachcomb-drive includes a rotor comb electrode and a stator comb electrodethat form a capacitive element that has a capacitance that depends onthe deflection angle of the MEMS scanning structure;

a driver configured to generate the at least one driving signal;

a sensing circuit selectively coupled to at least a subset of theplurality of comb-drives for receiving sensing signals therefrom,wherein each sensing signal is representative of the capacitance of acorresponding comb-drive; and

a processing circuit configured to detect and identify a parasitic modeof movement of the MEMS scanning structure based on the sensing signals.

One or more embodiments provide a scanning system, including:

microelectromechanical system (MEMS) scanning structure configured torotate about an axis with a desired rotational mode of movement based onat least one driving signal;

a plurality of comb-drives configured to drive the MEMS scanningstructure about the axis according to the desired rotational mode ofmovement based on the at least one driving signal, wherein eachcomb-drive includes a rotor drive-comb electrode and a stator drive-combelectrode;

a driver configured to generate the at least one driving signal;

a plurality of sensing combs, wherein each sensing comb includes a rotorsense-comb electrode and a stator sense-comb electrode that form acapacitive element that has a capacitance that depends on the deflectionangle of the MEMS scanning structure;

a sensing circuit selectively coupled to at least a subset of theplurality of sensing combs for receiving sensing signals therefrom,wherein each sensing signal is representative of the capacitance of acorresponding sensing combs; and

a processing circuit configured to detect and identify a parasitic modeof movement of the MEMS scanning structure based on the sensing signals.

One or more embodiments provide a scanning system, including:

microelectromechanical system (MEMS) scanning structure configured torotate about an axis with a desired rotational mode of movement based onat least one driving signal;

a plurality of combs-drives configured to drive the MEMS scanningstructure about the axis according to the desired rotational mode ofmovement based on the at least one driving signal, wherein eachcomb-drive includes a rotor comb electrode and a stator comb electrodethat form a capacitive element that has a capacitance that depends onthe deflection angle of the MEMS scanning structure;

a driver configured to generate the at least one driving signal;

a system controller configured to shift a driving frequency of the atleast one driving signal to provoke a parasitic mode coupling betweenthe desired rotational mode of movement and a parasitic mode of movementof the MEMS scanning structure;

a sensing circuit selectively coupled to at least a subset of theplurality of comb-drives for receiving sensing signals therefrom,wherein each sensing signal is representative of the capacitance of acorresponding comb-drive; and

a processing circuit configured to determine a frequency range of thedriving frequency at which the parasitic mode coupling occurs,

wherein the system controller controls the at least one driving signalto avoid the parasitic mode.

One or more embodiments provide a scanning system, including:

microelectromechanical system (MEMS) scanning structure configured torotate about an axis with a desired rotational mode of movement based onat least one driving signal;

a plurality of comb-drives configured to drive the MEMS scanningstructure about the axis according to the desired rotational mode ofmovement based on the at least one driving signal, wherein eachcomb-drive includes a rotor drive-comb electrode and a stator drive-combelectrode;

a driver configured to generate the at least one driving signal;

a plurality of sensing combs, wherein each sensing comb includes a rotorsense-comb electrode and a stator sense-comb electrode that form acapacitive element that has a capacitance that depends on the deflectionangle of the MEMS scanning structure;

a system controller configured to shift a driving frequency of the atleast one driving signal to provoke a parasitic mode coupling betweenthe desired rotational mode of movement and a parasitic mode of movementof the MEMS scanning structure;

a sensing circuit selectively coupled to at least a subset of theplurality of sensing combs for receiving sensing signals therefrom,wherein each sensing signal is representative of the capacitance of acorresponding sensing comb; and

a processing circuit configured to determine a frequency range of thedriving frequency at which the parasitic mode coupling occurs,

wherein the system controller controls the at least one driving signalto avoid the parasitic mode.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are described herein making reference to the appendeddrawings.

FIG. 1A is a schematic diagram of a LIDAR scanning system in accordancewith one or more embodiments;

FIG. 1B shows a schematic top view of an example of a mirror device inaccordance with one or more embodiments;

FIG. 1C shows a schematic bottom view of an example of the mirror deviceshown in FIG. 1B in accordance with one or more embodiments;

FIG. 1D illustrates a cross-sectional view of a MEMS mirror and driverelectrodes in accordance with one or more embodiments;

FIG. 2 is a schematic block diagram of the LIDAR scanning system inaccordance with one or more embodiments;

FIG. 3 illustrates a signal diagram of various signals generated by aMEMS driver based on a mirror angle θ and/or position according to oneor more embodiments;

FIG. 4A illustrates a top view of a MEMS mirror arranged in a nominalcentered position (left) and a translational shifted position (right)according to one or more embodiments;

FIG. 4B is a schematic diagram of a translational mode Ty measurementand mode damping system according to one or more embodiments;

FIG. 4C is a schematic diagram of a parasitic mode measurement and modedamping system according to one or more embodiments;

FIG. 4D illustrates a top view of a MEMS mirror being compensated with adamping voltage in response to detecting a translational movement Tyaccording to one or more embodiments;

FIG. 4E shows a timing diagram for implementing the first damping method(method 1) to damp parasitic mode with Ty according to one or moreembodiments;

FIG. 4F shows a timing diagram for implementing the second dampingmethod (method 2) to damp parasitic mode Tz according to one or moreembodiments;

FIG. 5A illustrates a top view of a MEMS mirror arranged in a nominalcentered position (left) and a yaw shifted position (right) according toone or more embodiments;

FIG. 5B is a schematic diagram of a yaw mode Rz measurement andcompensation system according to one or more embodiments;

FIG. 5C illustrates a top view of a MEMS mirror being compensated with adamping voltage in response to detecting a yaw movement Rz according toone or more embodiments;

FIGS. 6A and 6B illustrate cross-sectional views of a MEMS mirror anddriver electrodes in accordance with one or more embodiments;

FIG. 6C is a schematic diagram of a Tz mode measurement and compensationsystem according to one or more embodiments;

FIG. 6D illustrates a cross-sectional view of a MEMS mirror beingcompensated with a damping voltage in response to detecting a Tzmovement according to one or more embodiments.

FIG. 6E depicts four capacitor currents DL1R, DL2R, DL1L, and DL2L in anexcitation frequency sweep over a wide range from 4 kHz to 45 kHz for aMEMS mirror according one or more embodiments.

FIG. 7A illustrates a top view of a quasi-static (QS) MEMS mirrorarranged in a nominal centered position (left) and a translationalshifted position (right) according to one or more embodiments;

FIG. 7B is a schematic diagram of a Ty mode measurement and compensationsystem for a QS MEMS mirror according to one or more embodiments.

FIG. 7C illustrates a top view of a QS MEMS mirror being compensatedwith a damping voltage in response to detecting a translational movementTy according to one or more embodiments;

FIG. 7D illustrates a cross-sectional view of a QS MEMS mirror beingcompensated with a damping voltage in response to detecting atranslational movement Ty according to one or more embodiments;

FIG. 7E illustrates a top view of a QS MEMS mirror being compensatedwith an alternative method using dedicated sensing combs to detect atranslational movement Ty according to one or more embodiments; and

FIG. 8 illustrates a flow diagram of a parasitic mode coupling avoidancemethod according to one or more embodiments.

DETAILED DESCRIPTION

In the following, various embodiments will be described in detailreferring to the attached drawings. It should be noted that theseembodiments serve illustrative purposes only and are not to be construedas limiting. For example, while embodiments may be described ascomprising a plurality of features or elements, this is not to beconstrued as indicating that all these features or elements are neededfor implementing embodiments. Instead, in other embodiments, some of thefeatures or elements may be omitted, or may be replaced by alternativefeatures or elements. Additionally, further features or elements inaddition to the ones explicitly shown and described may be provided, forexample conventional components of sensor devices.

Features from different embodiments may be combined to form furtherembodiments, unless specifically noted otherwise. Variations ormodifications described with respect to one of the embodiments may alsobe applicable to other embodiments. In some instances, well-knownstructures and devices are shown in block diagram form rather than indetail in order to avoid obscuring the embodiments.

Further, equivalent or like elements or elements with equivalent or likefunctionality are denoted in the following description with equivalentor like reference numerals. As the same or functionally equivalentelements are given the same reference numbers in the figures, a repeateddescription for elements provided with the same reference numbers may beomitted. Hence, descriptions provided for elements having the same orlike reference numbers are mutually exchangeable.

Connections or couplings between elements shown in the drawings ordescribed herein may be wire-based connections or wireless connectionsunless noted otherwise. Furthermore, such connections or couplings maybe direct connections or couplings without additional interveningelements or indirect connections or couplings with one or moreadditional intervening elements, as long as the general purpose of theconnection or coupling, for example to transmit a certain kind of signalor to transmit a certain kind of information, is essentially maintained.

The term “substantially” may be used herein to account for smallmanufacturing tolerances (e.g., within 5%) that are deemed acceptable inthe industry without departing from the aspects of the embodimentsdescribed herein.

Directional terminology, such as “top”, “bottom”, “left”, “right”,“above”, “below”, “front”, “back”, “behind”, “leading”, “trailing”,“over”, “under”, etc., may be used with reference to the orientation ofthe figures and/or elements being described. Because the embodiments canbe positioned in a number of different orientations, the directionalterminology is used for purposes of illustration and is in no waylimiting. In some instances, directional terminology may be exchangedwith equivalent directional terminology based on the orientation of anembodiment so long as the general directional relationships betweenelements, and the general purpose thereof, is maintained.

In the present disclosure, expressions including ordinal numbers, suchas “first”, “second”, and/or the like, may modify various elements.However, such elements are not limited by the above expressions. Forexample, the above expressions do not limit the sequence and/orimportance of the elements. The above expressions are used merely forthe purpose of distinguishing an element from the other elements. Forexample, a first box and a second box indicate different boxes, althoughboth are boxes. For further example, a first element could be termed asecond element, and similarly, a second element could also be termed afirst element without departing from the scope of the presentdisclosure.

Embodiments relate to optical sensors and optical sensor systems and toobtaining information about optical sensors and optical sensor systems.A sensor may refer to a component which converts a physical quantity tobe measured to an electric signal, for example a current signal or avoltage signal. The physical quantity may, for example, compriseelectromagnetic radiation, such as visible light, infrared (IR)radiation, or other type of illumination signal, a current, or avoltage, but is not limited thereto. For example, an image sensor may bea silicon chip inside a camera that converts photons of light comingfrom a lens into voltages. The larger the active area of the sensor, themore light that can be collected to create an image.

A sensor device as used herein may refer to a device which comprises asensor and further components, for example biasing circuitry, ananalog-to-digital converter or a filter. A sensor device may beintegrated on a single chip, although in other embodiments a pluralityof chips or also components external to a chip may be used forimplementing a sensor device.

In Light Detection and Ranging (LIDAR) systems, a light source transmitslight pulses into a field of view and the light reflects from one ormore objects by backscattering. In particular, LIDAR is a directTime-of-Flight (ToF) system in which the light pulses (e.g., laser beamsof infrared light) are emitted into the field of view, and a pixel arraydetects and measures the reflected beams. For example, an array ofphotodetectors receives reflections from objects illuminated by thelight.

Differences in return times for each light pulse across multiple pixelsof the pixel array can then be used to make digital 3D representationsof an environment or to generate other sensor data. For example, thelight source may emit a single light pulse, and a time-to-digitalconverter (TDC) electrically coupled to the pixel array may count fromthe time the light pulse is emitted, corresponding to a start signal,until a time the reflected light pulse is received at the receiver(i.e., at the pixel array), corresponding to a stop signal. The“time-of-flight” of the light pulse is then translated into a distance.

In another example, an analog-to-digital converter (ADC) may beelectrically coupled to the pixel array (e.g., indirectly coupled withintervening elements in-between) for pulse detection and ToFmeasurement. For example, an ADC may be used to estimate a time intervalbetween start/stop signals with an appropriate algorithm. For example,an ADC may be used to detect an analog electrical signal from one ormore photodiodes to estimate a time interval between a start signal(i.e., corresponding to a timing of a transmitted light pulse) and astop signal (i.e., corresponding to a timing of receiving an analogelectrical signal at an ADC) with an appropriate algorithm.

A scan such as an oscillating horizontal scan (e.g., from left to rightand right to left of a field of view) or an oscillating vertical scan(e.g., from bottom to top and top to bottom of a field of view) canilluminate a scene in a continuous scan fashion. Each firing of thelaser beam by the light sources can result in a scan line in the “fieldof view.” By emitting successive light pulses in different scanningdirections, an area referred to as the field of view can be scanned andobjects within the area can be detected and imaged. Thus, the field ofview represents a scanning plane having a center of projection. A rasterscan could also be used.

FIG. 1A is a schematic diagram of a LIDAR scanning system 100 inaccordance with one or more embodiments. The LIDAR scanning system 100is an optical scanning device that includes a transmitter, including anillumination unit 10, a transmitter optics 11, and a one-dimensional(1D) microelectromechanical system (MEMS) mirror 12, and a receiver,including a second optical component 14 and a photodetector detectorarray 15.

The illumination unit 10 includes multiple light sources (e.g., laserdiodes or light emitting diodes) that are linearly aligned in single barformation and are configured to transmit light used for scanning anobject. The light emitted by the light sources is typically infraredlight although light with another wavelength might also be used. As canbe seen in the embodiment of FIG. 1A, the shape of the light emitted bythe light sources is spread in a direction perpendicular to thetransmission direction to form a light beam with an oblong shapeperpendicular to a transmission. The illumination light transmitted fromthe light sources are directed towards the transmitter optics 11configured to focus each laser onto a one-dimensional MEMS mirror 12.The transmitter optics 11 may be, for example, a lens or a prism.

When reflected by the MEMS mirror 12, the light from the light sourcesis aligned vertically to form for each emitted laser shot aone-dimensional vertical scanning line SL of infrared light or avertical bar of infrared light. Each light source of the illuminationunit 10 contributes to a different vertical region of the verticalscanning line SL. Thus, the light sources may be concurrently activatedand concurrently deactivated to obtain a light pulse with multiplesegments, where each segment corresponds to a respective light source,However, each vertical region or segment of the vertical scanning lineSL may also be independently active or inactive by turning on or off acorresponding one of the light sources of the illumination unit 10.Thus, a partial or full vertical scanning line SL of light may be outputfrom the system 100 into the field of view.

Accordingly, the transmitter of the system 100 is an optical arrangementconfigured to generate laser beams based on the laser pulses, the laserbeams having an oblong shape extending in a direction perpendicular to atransmission direction of the laser beams.

In addition, while three laser sources are shown, it will be appreciatedthat the number of laser sources are not limited thereto. For example,the vertical scanning line SL may be generated by a single laser source,two laser sources or more than three laser sources.

The MEMS mirror 12 is a mechanical moving mirror (i.e., a MEMSmicro-mirror) formed in a semiconductor substrate. The MEMS mirror 12according to this embodiment is suspended by mechanical springs (e.g.,leaf springs, sometimes referred to as cantilever beams) or flexures andis configured to rotate about a single axis and can be said to have onlyone desired degree of freedom for movement (i.e., a desired rotationalmode about the single axis). While other degrees of freedom may exist,e.g., motions associated with other rigid body degrees of freedom, thesemodes are undesired and are referred to as unwanted parasitic modes. Oneobject of the disclosed embodiments is to suppress the parasitic modesthat are associated with these other, unwanted degrees of freedom.

The parasitic/unwanted oscillations are described as the excitation ofat least one of the rigid body modes of the MEMS mirror 12 differentfrom the desired operational mode (i.e., the desired rotational modeabout the single axis). The parasitic/unwanted oscillations can occurdue to a direct excitation of parasitic modes via the actuators (e.g.,electrostatic comb-drives, electromagnetic drive, etc.) or high orderharmonic motions of the main mode by nonlinear oscillation, or due to anindirect excitation of parasitic modes via mode coupling mechanisms.

Direct excitation of parasitic modes via the actuators may be caused by,in resonant mirrors, higher harmonic content of the periodic drivesignal fulfilling a condition for parametric resonance of a parasiticmode, or, in quasi-static mirrors, switching of drive voltage for changeof scan angle will excite ringing of the parasitic mode because thecomb-drive not only induces the intended rotation, but also createsother forces, e.g., y-force. Thus, switching the angle of a quasi-stateMEMS mirror will lead to transients of all modes, including desired andunwanted modes.

Indirect excitation of the parasitic modes via mode coupling mechanismsmay be caused by an Euler force that couples the desired Rx mode toparasitic mode Ty. The Euler force may be due to an offset of a centerof mass and the rotation axis 13. Indirect excitation of the parasiticmodes via mode coupling mechanisms may be caused by a centrifugal forcethat couples the desired Rx mode to parasitic mode Tz. The centrifugalforce may be due to an offset of a center of mass and the rotation axis13. Indirect excitation of the parasitic modes via mode couplingmechanisms may be caused by a coupling of all rotational modes Rx, Ry,and Rz by Euler's equations. The nonlinearity of the oscillation, e.g.by geometric hardening, can contain specific frequency components ofother parasitic modes, cause excitation. Also possible, so-calledautoparametric excitation and/or three-wave-mixing-coupling, both ofthem a consequence of geometric nonlinearities induced by largedeflections.

In order to make a MEMS scanning mirror robust against vibrations, themirror should have a low inertia, i.e., a light mirror body, but stiffenough to keep dynamic deformations acceptable. In addition, the mirrorshould have a high stiffness of its suspension for all parasiticdegrees-of-freedom (DOF) of the mirror body. The stiffness associatedwith the rotation about the operational axis should be designed to matchthe desired operation frequency.

In order to achieve a light and stiff mirror body, the mirror body maycomprise a relatively thin mirror and a thicker reinforcement structurefor the mirror. The mirror body may be rotatably arranged in a mirrorframe around a rotation axis (i.e., a scanning axis) extending in aplane defined by the mirror frame (e.g., in an x-y plane). In thefollowing examples, it will be assumed that the rotation axis extendsparallel to the x-dimension of the x-y plane. The rotation axis mayextend to first and second mutually opposite end-portions of the mirrorbody. The z-axis location of the rotation axis is mainly defined by thez-axis extension of the springs and flexures. The mirror may have areflective plane on a first main surface and opposite the first mainsurface a second main surface provided with the reinforcement structure,which generally have larger thicknesses extending in z-direction thanthe flexures and springs. Thus, the reinforcement structure is displacedin the z-dimension from both the first main surface of the mirror bodyand the rotation axis. For this reason, also the center of mass of theentire rotating body is generally displaced in the z-dimension from therotation axis.

In order to achieve a high stiffness suspension, the mirror body may besupported in the mirror frame using support beams extending along therotation axis and additional cantilever beam or leaf spring assemblies,as illustrated in FIG. 1B. Generally, as defined herein, leaf springassemblies may be referred to as cantilever beam assemblies, and viceversa. Similarly, leaf springs and cantilever beams may be usedinterchangeably.

A cantilever beam assembly may have a longitudinal direction and mayextend within the plane defined by the frame. The support beams may beconnected between the mirror body and the frame at two opposite ends ofthe mirror body along the rotation axis. The cantilever beam assemblymay have a cantilever beam coupled at a first end via a relief structureto the mirror frame and fixed at a second end to the mirror body. Thecantilever beam may have a thickness, perpendicular to a plane of theframe, that is smaller than its width in the plane of the frame.

Results of the low inertia and the high suspension stiffness of themirror body may be high resonance frequencies and a good dynamicperformance. These properties may also make the device, which isoperated at the resonance frequency around the main axis of rotation,very fast. In normal operation, i.e., at resonance, accelerations at themirror tips of typically 10000 G may be achieved.

The MEMS mirror 12 may be assembled in a chip package 27 shown in FIG. 2to protect the mirror. For example, the MEMS mirror 12 may behermetically sealed at a low pressure (i.e., at a pressure lower thanatmospheric pressure) in a chip package. This low pressure may provide alow damping environment in which the MEMS mirror 12 operates.

Conceivable packages may include or differ by one or more of thefollowing variants: different substrates (e.g., metal (leadframe),ceramic, organic (similar to printed circuit board (PCB) material)), anddifferent optical lids or covers (e.g., optical material of glass,silicon, sapphire, etc.). Furthermore, the optical lids or covers may becavity-forming caps, may be integrated into a frame (e.g., a metalframe), or assembled onto a pre-mold cavity or a ceramic cavity.

One or more methods (e.g., adhesive bonding, gluing, soldering, welding,and the like) or one or more different materials (e.g., silicone, glasssolder, AuSn, and the like) may be used to bond one or more elementstogether (e.g., joining cap or lid to substrate). It will be appreciatedthat bonding methods may be interchangeable across various embodimentsdisclosed herein.

Alternatively, a wafer-level approach may be used such that acavity-shaped lid may be directly mounted onto the MEMS chip (or even onwafer-level prior to singulation). Here, if the lid attachment leavesthe electrical pads exposed, the sub-mount chip/lid can further beprocessed into a package using molding or casting processes.

The MEMS mirror 12 is a mechanical moving mirror (i.e., a MEMSmicro-mirror) integrated on a semiconductor chip (not shown). The MEMSmirror 12 according to this embodiment is configured to rotate about asingle scanning axis and can be said to have only one degree of freedomfor scanning because other degrees of freedom, e.g. motions associatedwith other rigid body degrees of freedom, are significantly suppressedcompared to the said rotation about the single axis. This is reflectedby significantly higher eigenfrequencies of vibration associated withthe other degrees of freedom compared to the said rotational motionabout the single axis. Distinguished from 2D-MEMS mirrors (2D MEMSscanners), in the 1D MEMS mirror, the single scanning axis is fixed to anon-rotating substrate and therefore maintains its spatial orientationduring the oscillation of the MEMS mirror. Thus, a 1D oscillating MEMSmirror is by design more robust against vibrations and shocks than 2DMEMS mirror solutions. Due to this single scanning axis of rotation, theMEMS mirror 12 is referred to as a 1D MEMS mirror or 1D MEMS scanner.While embodiments described herein use 1D oscillating MEMS mirrors, themethods described herein can be extended to 2D MEMS mirrors as well. Inthis case, both axes of a single 2D MEMS mirror are controlled bydifferent controller (in case of resonant MEMS with phase-locked loops(PLLs)) such that a first scanning direction of the 2D MEMS mirrorsaccording to a first axis is synchronized according to any of thesynchronization techniques described herein, and a second scanningdirection of the 2D MEMS mirrors according to a second axis issynchronized according to any of the synchronization techniquesdescribed herein. It is further possible that the different controller(e.g. PLLs) are provided in separate MEMS drivers or integrated into asingle MEMS driver for both axes of the 2D MEMS mirror.

The MEMS mirror 12 itself can be a statically tiltable mover oroscillator (quasi-static MEMS mirror or resonator) to move or oscillate“side-to-side” about a single scanning axis 13 following a staticposition, a trajectory, or a resonant oscillation. For example, the MEMSmirror 12 may be either a resonant mirror or a quasi-static (QS) mirror.The MEMS mirror 12 is configured to move or oscillate “side-to-side”about the single scanning axis 13 such that the light reflected from theMEMS mirror 12 (i.e., the vertical scanning line of light) moves in ahorizontal scanning direction.

As a resonant mirror, the MEMS mirror 12 can be a non-linear resonatorthat feature nonlinear properties due to the increasing stiffness of thesuspension. In particular, the MEMS mirror 12 may exhibit a non-linearbehavior due the torsional stiffness about the scanning axis 13 providedby the leaf spring assemblies 30 which is not constant, but increaseswith increasing angle. A consequence of this so-called geometrichardening is that an oscillation frequency of the mirror increases withan increase in oscillation amplitude (i.e., deflection angle amplitude0). Thus, the stiffening of the suspension causes the MEMS mirror 12 toexhibit nonlinear properties.

It is noted that the deflection angle θ of the MEMS mirror 12 about thescanning axis 13 may be referred to as a tilt angle, rotation angle,scanning angle, mirror angle θ_(mirror) or θ_(m), or the like, and thatthese terms be used interchangeably throughout the disclosure.

As a QS MEMS mirror, the MEMS mirror 12 can tilt statically, followtrajectories (e.g., step, triangle, and sawtooth shaped trajectories) orresonate at its resonance frequency (i.e., its eigenfrequency). The QSMEMS usually has a weak nonlinear to no nonlinear frequency-amplitudedependence as the torsional stiffness is much more linear.

A scanning period or an oscillation period is defined, for example, byone complete oscillation from a first edge of the field of view (e.g.,left side) to a second edge of the field of view (e.g., right side) andthen back again to the first edge. A mirror period of the MEMS mirror 12corresponds to a scanning period.

Thus, the field of view is scanned in the horizontal direction by thevertical bar of light by changing the angle θ of the MEMS mirror 12 onits scanning axis 13. For example, the MEMS mirror 12 may be configuredto oscillate at a resonance frequency of 2 kHz, between +/−15 degrees tosteer the light over +/−30 degrees making up the scanning range of thefield of view. Thus, the field of view may be scanned, line-by-line, bya rotation of the MEMS mirror 12 about the axis of its degree of motion.One such sequence through the degree of motion (e.g., from −15 degreesto +15 degrees) is referred to as a single scan or scanning cycle.Multiple scans may be used to generate distance and depth maps, as wellas 3D images by a processing unit.

While the transmission mirror is described in the context of a MEMSmirror, it will be appreciated that other 1D mirrors can also be used.In addition, the resonance frequency or the degree of rotation is notlimited to 2 kHz and +/−15 degrees, respectively, and both the resonancefrequency and the field of view may be increased or decreased accordingto the application. Thus, a one-dimensional scanning mirror isconfigured to oscillate about a single scanning axis and direct thelaser beams at different directions into a field of view. Hence, atransmission technique includes transmitting the beams of light into thefield of view from a transmission mirror that oscillates about a singlescanning axis such that the beams of light are projected as a verticalscanning line SL into the field of view that moves horizontally acrossthe field of view as the transmission mirror oscillates about the singlescanning axis.

Upon impinging one or more objects, the transmitted bar of verticallight is reflected by backscattering back towards the LIDAR scanningsystem 100 as a reflected vertical line where the second opticalcomponent 14 (e.g., a lens or prism) receives the reflected light. Thesecond optical component 14 directs the reflected light onto thephotodetector detector array 15 that receives the reflected light as areceiving line RL and is configured to generate electrical measurementsignals. The electrical measurement signals may be used for generating a3D map of the environment and/or other object data based on thereflected light (e.g., via ToF calculations and processing).

The receiving line is shown as a vertical column of light that extendsalong one of the pixel columns in a lengthwise direction of the pixelcolumn. The receiving line has three regions that correspond to thevertical scanning line SL shown in FIG. 1A. As the vertical scanningline SL moves horizontally across the field of view, the vertical columnof light RL incident on the 2D photodetector array 15 also moveshorizontally across the 2D photodetector array 15. The reflected lightbeam RL moves from a first edge of the photodetector detector array 15to a second edge of the photodetector detector array 15 as the receivingdirection of the reflected light beam RL changes. The receivingdirection of the reflected light beam RL corresponds to a transmissiondirection of the scanning line SL.

The photodetector array 15 can be any of a number of photodetectortypes; including avalanche photodiodes (APD), silicon photomultipliers(SiPMs) photocells, and/or other photodiode devices. Imaging sensorssuch as charge-coupled devices (CCDs) can be the photodetectors. In theexamples provided herein, the photodetector array 15 is atwo-dimensional (2D) APD array that comprises an array of APD pixels. Inother embodiments, the photodetector array 15 may be a 1D array thatincludes a single column of photodiodes. The activation of thephotodiodes may be synchronized with light pulses emitted by theillumination unit 10. Alternatively, a single photo detector cell/pixel,as opposed to an array, may be used. For example, a single photodetector cell/pixel may be used in case of a 2×1D scanning transmitterin a coaxial LIDAR architecture.

The photodetector array 15 receives reflective light pulses as thereceiving line RL and generates electrical signals in response thereto.Since the time of transmission of each light pulse from the illuminationunit 10 is known, and because the light travels at a known speed, atime-of-flight computation using the electrical signals can determinethe distance of objects from the photodetector array 15. A depth map canplot the distance information.

In one example, for each distance sampling, a microcontroller triggers alaser pulse from each of the light sources of the illumination unit 10and also starts a timer in a Time-to-Digital Converter (TDC) IntegratedCircuit (IC). The laser pulse is propagated through the transmissionoptics, reflected by the target field, and captured by an APD of the APDarray 15. The APD emits a short electrical pulse which is then amplifiedby an electrical signal amplifier. A comparator IC recognizes the pulseand sends a digital signal to the TDC to stop the timer. The TDC uses aclock frequency to calibrate each measurement. The TDC sends the serialdata of the differential time between the start and stop digital signalsto the microcontroller, which filters out any error reads, averagesmultiple time measurements, and calculates the distance to the target atthat particular field position. By emitting successive light pulses indifferent directions established by the MEMS mirror, an area (i.e., afield of view) can be scanned, a three-dimensional image can begenerated, and objects within the area can be detected.

Alternatively, instead of using the TDC approach, ADCs may be used forsignal detection and ToF measurement. For example, each ADC may be usedto detect an analog electrical signal from one or more photodiodes toestimate a time interval between a start signal (i.e., corresponding toa timing of a transmitted light pulse) and a stop signal (i.e.,corresponding to a timing of receiving an analog electrical signal at anADC) with an appropriate algorithm.

It will be appreciated that the above-described horizontal scanningsystem 100 may also be used for vertical scanning. In this case, thescanning arrangement is arranged such that the scanning direction isrotated 90° such that the scanning line SL and the receiving line RLmove in the vertical direction (i.e., from top to bottom or from bottomto top). As such, the scanning line is a horizontal scanning line SLthat is projected into the field of view that moves vertically acrossthe field of view as the transmission mirror oscillates about the singlescanning axis. Furthermore, as the horizontal scanning line SL movesvertically across the field of view, the horizontal column of light RLincident on the 2D photodetector array 15 also moves vertically acrossthe 2D photodetector array 15.

It will be further appreciated that a LIDAR scanning system may includesynchronized MEMS mirrors that are used in a 2×1D system, such as aLissajous scanning system. In this case, the MEMS mirrors are mounted inthe same location in the vehicle and are configured to scan the twodimensions (horizontal and vertical) of a common field of view.

FIG. 1B shows a schematic top view of an example of a mirror device inaccordance with one or more embodiments. Referring to FIG. 1B, anexample of a mirror device, such as a MEMS scanning micro mirror, is nowexplained. The mirror device comprises a mirror body 8. The mirror body8 comprises a mirror 12 and a mirror support 16. The mirror devicefurther includes a frame 17. The mirror body 8 is arranged in the frame17. The frame 17 defines a plane, i.e., the (x, y) plane in FIG. 1B. Theplane defined by the frame 17 may be parallel to planes defined by mainsurfaces of a layer or a plurality of layers in which the frame 17 isformed.

The mirror body 8 is rotatable around a scanning axis 13 extending inthe plane defined by the frame 17. Support beams 18, which may also bereferred to as torsion beams, are connected between the mirror body 8and the frame 17 along the scanning axis 13. To be more specific, afirst support beam 18 is connected between a first end of the mirrorbody 8 and the frame 17 and a second support beam 18 is connectedbetween a second end of the mirror body 8 and the frame 17, where thesecond end of the mirror body 8 is opposite to the first end in thedirection of the scanning axis 13. An enlarged view of one of thesupport beams 18 is shown in the enlarged portion C in the right handside of FIG. 1B. As can be seen, support beams 18 connect parts ofmirror support 16 to parts of frame 17 and permit the mirror body 8 tobe rotated around scanning axis 13. The support beams 18 may becollinear with the scanning axis 13.

Those skilled in the art will appreciate that the shape of the mirror 12can be any shape desired for a particular application, e.g., a circle,ellipse, square, rectangle or other shape as desired.

The mirror frame 17 defines a mirror recess 20 in which the mirror body8 is arranged. The mirror recess 20 is defined by a recess periphery 28of the mirror frame 17. The mirror frame 17 may also be structured todefine further recesses in which other components may be arranged, suchas actuators, sensors and leaf spring assemblies.

The mirror device can include a leaf spring assembly 30. In the exampleshown, the mirror device includes two pairs of leaf spring assemblies30, where the leaf spring assembly in each pair extends from the mirrorbody 8 in opposite directions. In the example shown, the leaf springassemblies 30 are arranged symmetrically with respect to the scanningaxis 13. However, for QS MEMS mirrors, the leaf-springs are not used.Instead, torsion bars (or equivalent structures with several torsionbars, e.g., V-shape, PI-shape) support the MEMS mirror.

The at least one leaf spring assembly 30 includes a leaf spring 32 and arelief link 34. The relief link 34 may have one or more relief springs35. The leaf spring 32 includes a first end 32 a and a second end 32 b.The first end 32 a is coupled to the mirror body 8 and the second end iscoupled to the frame 17. Each leaf spring 32 has a longitudinaldirection or extension between the first end 32 a and the second end 32b. The first end 32 a is fixed to the mirror support (not illustrated)and the second end 32 b is coupled to frame 17 via the relief link 34.In the examples, the first ends 32 a of two leaf springs 32 extendingfrom the same portion of the mirror body 8 in different directions maybe connected to each other (e.g., the leaf springs of the left side ofthe mirror 12 or the leaf springs on the right side of the mirror 12),stiffened only in the vicinity of the scanning axis 13 by areinforcement structure (not illustrated) such that the two leaf springs32 can exert torque on the mirror body 8.

In some examples, the shape of the mirror 12 may include concaveportions in the region of the scanning axis 13, wherein portions of theleaf springs 32 extend into the concave portions of the mirror 12. Insome examples, leaf springs 32 and mirror 12 may be formed in a samelayer of material and may be connected to each other adjacent thescanning axis 13.

In some examples, the leaf springs 32 may be implemented in a singlecrystal silicon layer having a direction of lower material stiffness,where the leaf springs have their longitudinal direction aligned withthe direction of lower material stiffness. In some examples, the leafsprings 32 may be implemented in a silicon layer having a <100> axis andthe leaf springs have their longitudinal direction aligned with the<100> direction which in this case has the lower material stiffness.

Torsional stiffness about the scanning axis 13 may be set using the leafspring assemblies 30. The pair of support beams 18 supports the mirrorbody 8 vertically, i.e., perpendicular to a main surface of the frame17, at the scanning axis 13. However, the support beams 18 may have anegligible effect on the torsional stiffness, so that the naturalfrequency of the mirror body may be substantially determined by the leafspring assemblies 30. The natural frequency may be substantiallyindependent of the support beams 18. The natural frequency as definedherein is the undamped frequency of the eigenmode of the mirror body 8(i.e., the mirror 12) about its scanning axis 13 at small angles. Thesupport beams 18 may define the out-of-plane rocking and verticaltranslational mode stiffness for the corresponding rigid body modes aswell as the corresponding eigenfrequencies. The torsional stiffness canbe decoupled from the out-of-plane rocking and vertical translationalmode stiffness so that the out-of-plane rocking and verticaltranslational mode frequencies can be set to desired values, such ashigher values, without influencing the torsional mode stiffness andresonance frequency.

As defined herein, the X axis is along the scanning axis 13, the Y axisis perpendicular to the X axis on the mirror plane when the mirror 12 isat rest, and the Z axis is perpendicular to and out of the mirror planewhen the mirror 12 is at rest. The X, Y, and Z axis are axes of athree-dimensional Cartesian coordinate system.

In the example shown in FIG. 1B, one end of the at least one leaf spring32 is connected to the mirror body 8 at a location close to the scanningaxis 13. The other end 32 b is connected to the associated relief link34 at a location further away from the scanning axis 13. The leaf springassemblies 30 may provide torsional stiffness to the mirror body 8 aboutthe scanning axis 13. The relief links 34 may provide a compliant orflexible coupling from the leaf springs 32 to the frame 17. The relieflinks 34 may have a relatively low stiffness longitudinal to the leafsprings 32, i.e., in Y direction in FIG. 1B, which allows one end of theleaf springs 32 to move in their longitudinal direction when the mirrorbody 8 rotates around the scanning axis 13. The relief links 34 may havea relatively high stiffness in the transverse direction, i.e., in Zdirection and in X direction in FIG. 1B.

The resonance frequency for rotation of the mirror 12 around thescanning axis 13 may be defined mainly by the inertia of the mirror body8 and the stiffness of the leaf spring assemblies 30, which may bedefined by the bending stiffness of the leaf springs 32 and by thetorsional and translational stiffness of the relief links 34. Thebending stiffness of the leaf springs 32 may be defined by the length,width, and, in particular, the thickness of the leaf springs 32. Thecombined stiffness in Y direction of the support beams 18 and the relieflinks 34 may prevent movement of the mirror body 8 perpendicular to thescanning axis 13 (in the X direction) during operation. More detail onthe relief links is provided below.

The support beams 18 are connected between the frame 17 and the mirrorbody 8 along the scanning axis 13 to support the mirror body 8 in theframe 17. In one example, the support beams 18 have narrow rectangularcross-sections perpendicular to the scanning axis 13, with the long axisof the rectangle perpendicular to the face of the mirror 12 and themirror body 8, and the short axis of the rectangle parallel to the faceof the mirror 12. The torsional stiffness corresponding to a rotation ofthe mirror body 8 around scanning axis 13 may be provided by the leafspring assemblies 30. The support beams 18 may serve only for support ofthe mirror body 8 and may have a negligible effect on the torsionalstiff-ness. The support beams 18 may be sized so that the stiffnessagainst vertical translational displacement (in Z direction) of themirror body 8 and against its in-plane translation perpendicular to thescanning axis 13 (i.e., along the Y axis) may be as high as possible.

The mirror device may also include at least one actuator to providetorque to drive the mirror body 8 about the scanning axis 13. In oneexample, the actuator may be a comb-drive that includes mirror combsattached to the mirror body 8 interleaved with frame combs attached tothe frame 17. Applying a difference in electrical potential betweeninterleaved mirror combs and frame combs creates a driving force betweenthe mirror combs and the frame combs, which creates a torque on themirror body 8 about the scanning axis 13. An oscillating electricalpotential can be applied to resonantly excite the mirror device close toits natural frequency. In this example, four comb-drives 40TL (topleft), 40BL (bottom left), 40TR (top right), and 40BR (bottom right) areprovided. The left side is arranged to the left of the scanning axis 13,whereas the right side is arranged to the right of the scanning axis 13.Each comb-drive comprises a stator comb-drive electrode fixed to theframe 17 and a rotor comb-drive electrode that is movable with themirror body 8 as it rotates about the scanning axis 13.

In other examples, actuation methods may include electromagneticactuation and piezoelectric actuators. In electromagnetic actuation, themicro mirror may be “immersed” in a magnetic field and an alternatingelectric current through conductive paths on the mirror body may createthe oscillating torque around the scanning axis 13. Piezoelectricactuators may be integrated in the leaf springs or the leaf springs maybe made of piezoelectric material to produce alternating beam bendingforces in response to an electrical signal and generate the oscillatingtorque.

The desired rotational mode Rx of the MEMS mirror 12 about the scanningaxis 13 is shown in FIG. 1B. The bidirectional arrow corresponding tothe rotational mode Rx indicates that the MEMS mirror 12 is configuredto oscillate about the scanning axis 13 to perform a scanning operation.Unwanted parasitic modes are also shown. For example, an in-planetranslational mode Ty is shown that represents translational movement ofthe mirror body 8 in the Y direction. In particular, there is a resonantcoupling from the desired rotational mode Rx to a translational mode inY direction, Ty. The coupling of the mode Rx to the translational modeTy is predominantly caused by the reinforcement structures. In addition,a nonlinear coupling can occur from the desired rotational mode Rx to anunwanted spurious yaw mode Rz. Thus, the spurious yaw mode Rz results inthe mirror body 8 moving about the Z axis. Furthermore, the mode Rx cancouple to the translational mode in Z direction, Tz. This coupling of Rxto Tz is predominantly caused by the reinforcement structures. Thecoupling of Rx to Ty and Tz, respectively, originates from inertialforces, which are caused by a rotational unbalance since the center ofmass is displaced with respect to the rotation axis along the Z axis dueto the mass of the reinforcement structures.

FIG. 1C shows a schematic bottom view of an example of the mirror deviceshown in FIG. 1B in accordance with one or more embodiments. Inparticular, an underside of the mirror body 8 and the frame 17 areshown. The four comb-drives 40TL, 40BL, 40TR, and 40BR are also visible.Here, the reinforcement structure 9 that supports the mirror body 8 canbe seen.

FIG. 1D illustrates a cross-sectional view of a MEMS mirror and driverelectrodes in accordance with one or more embodiments. In particular,MEMS mirror 12 is shown rotating around a rotation axis 13 that extendsorthogonal to the page. The MEMS mirror 12 includes mirror combelectrodes 41 and 42, to which a drive voltage v_(drive) is applied. Asdescribed above, the MEMS mirror 12 is supported by a mirror frame 17(not illustrated). The mirror frame 17 includes static comb electrodes43 and 44. The mirror comb electrodes 41 and 42, coupled to opposingends of the mirror body 8, move along with the oscillation of the MEMSmirror 12. The drive voltage v_(drive) induces a driving force betweeninterdigitated mirror comb electrodes 41, 42 and the static combelectrodes 43, 44. In particular, the MEMS mirror 12 is driven inparametric resonance by a square wave voltage with 50% (or different)duty cycle.

A first drive capacitor CA (also referred to as CA(θ)) is formed by anoverlap of electrodes 41 and 44. The overlap of the two electrodes 41and 44 varies with the rotation angle θ_(mirror) of the MEMS mirror 12about the rotation axis 13. As the overlap increases, so does thecapacitance of the first drive capacitor CA. Conversely, the capacitanceof the first drive capacitor CA decreases as the overlap decreases.

Similarly, a second drive capacitor CB (also referred to as CB(θ)) isformed by an overlap of electrodes 42 and 43. The overlap of the twoelectrodes 42 and 43 varies with the rotation angle θ_(mirror) of theMEMS mirror 12 about the rotation axis 13. As the overlap increases, sodoes the capacitance of the second drive capacitor CB. Conversely, thecapacitance of the second drive capacitor CB decreases as the overlapdecreases.

Thus, the first drive capacitor CA is a function of the rotation angle θthat stores a first charge based on the rotation angle θ_(mirror) of theMEMS mirror 12 resulting in a first displacement current that is alsodependent on the rotation angle θ of the MEMS mirror 12. Similarly, thesecond drive capacitor CB is also a function of the rotation angleθ_(mirror) that stores a second charge based on the rotation angle ofthe MEMS mirror 12 resulting in a second displacement current that isalso dependent on the rotation angle of the MEMS mirror 12.

For the QS MEMS, the actuation is unipolar, this means for positivedeflection angle the one capacitor increases (immersed combs) and theother capacitance is around zero (not immersed combs). For negativedeflection it is vice versa. For resonant MEMS, the two drive capacitorseach have a symmetric capacitance characteristic according to thedeflection direction (e.g., the tilt direction) of the MEMS mirror 12.However, bipolar actuation may also be used, where a third capacitor isformed with the middle electrode (like resonant electrode).

More specifically, this means that with perfectly aligned combsCA(−θ_(mirror))=CA(θ_(mirror)) and CB(−θ_(mirror))=CB(θ_(mirror)), i.e.both capacitances are even functions with symmetric capacitancedependence, which do not change their value if the angle of the mirroris reversed. Because of design symmetry, CA(θ_(mirror))=CB(θ_(mirror))also holds true. It is also to be noted that while two drive capacitorsCA and CB are shown in the cross-sectional view, four drive capacitorsare present, as shown in FIGS. 1B and 1C. Thus, four displacementcurrents are generated based on the rotation angle θ_(mirror). Becauseof the symmetry of the capacitances for θ→−0, the scanning direction(e.g., clockwise or counter-clockwise) could not be detected for anideal and pure Rx rotational motion.

FIG. 1D further shows the mode coupling between the Rx mode and the Tymode caused by a displacement L between the center of mass M from therotational axis (scanning axis 13), due to the reinforcement structure9. The coupling force originates from the Euler force according toEquation 1:

f _(Y)(t)=mL{umlaut over (θ)} _(m)(t)  Eq. 1,

with the mirror mass m and the rotational acceleration {umlaut over(θ)}_(mirror)(t). Thus, the Ty mode is excited by the inertial forcef_(Y)(t) having the same frequency content as the Rx trajectory θ(t) byvirtue of the dependence on the rotational acceleration, which is thesecond derivative of the trajectory. Additionally, the Rx and Tyeigenmodes are not perfectly pure rotational and translation modes,respectively, meaning that a non-zero amplitude of the Ty mode also hasa small rotational admixture, i.e., a rotation about the X axis.

One or more embodiments use the change in the sensing signal (e.g., acurrent signal) of a subset of the comb-drive electrodes, caused by anexcited parasitic mode to detect the scanning direction (e.g., clockwiseor counter-clockwise) as well as the movement amplitude and phase ofparasitic modes, i.e., Ty, Tz, and Rz. A subset of comb-drive electrodesmay include any combination of left/right, top/bottom,frontside/backside electrodes.

The Euler force caused by the finite displacement L between center ofmass M and rotational axis 13 leads to a small excitation of Tysuperimposed onto the Rx motion. This superimposed motion removes thesymmetry of the displacement currents for θ and −θ such that the sign ofthe angle, i.e., the scanning direction, can be detected.

The subset of the comb-drive electrodes can be a subset of stator orrotor electrodes or in the 2D MEMS mirror case a subset of rotor, innerframe, and/or outer frame (i.e., actual stator). As an example,different cases are shown where two current signals are subtracted(e.g., a left current signal from a right current signal) to get thehighest efficiency. However, it could be also possible to detect themodes by only one side currents or even only the current of onecomb-drive, but with lower efficiency. Also, the sensing does not haveto be based on the current. It can be any method that provides a sensingsignal, which is related to a capacitance or capacitance change of oneor more of the drive capacitors (e.g., charge sensing, current sensing,or frequency modulation-based sensing).

After detection, the parasitic modes Ty, Tz, Ry, and Rz can be activelydamped. In particular, a system controller 23 can apply counteractingforce/torque by suitable selection of comb electrodes in any combination(either combs for common usage of actuation/sensing/damping or evenindividual combs for damping only). A suitable selection of combelectrodes means that the selection will have a force/torquecorresponding to the identified parasitic mode and can counteract themirror motion in this identified parasitic mode. Thus, the selection ofcomb electrodes to which a counteractive force (e.g., a damping voltage)is to be applied is based on the parasitic mode Ty, Tz, Ry, Rz, etc.that is targeted by the system controller 23 for damping based on itsdetection by the system controller. A switching network may be used bythe system controller 23 to apply the counteractive force to selectedcomb electrodes corresponding to the targeted parasitic mode at a timingthat is effective for damping the targeted parasitic mode. Thisparasitic mode detection, identification, and targeting damping will bedescribed in detail below.

FIG. 2 is a schematic block diagram of the LIDAR scanning system 200 inaccordance with one or more embodiments. In particular, FIG. 2 showsadditional features of the LIDAR scanning system 200, including exampleprocessing and control system components such as a MEMS driver, areceiver circuit, and a system controller.

The LIDAR scanning system 200 includes a transmitter unit 21 that isresponsible for an emitter path of the system 200, and a receiver unit22 that is responsible for a receiver path of the system 200. The systemalso includes a system controller 23 that is configured to controlcomponents of the transmitter unit 21 and the receiver unit 22, and toreceive raw data from the receiver unit 22 and perform processingthereon (e.g., via digital signal processing) for generating object data(e.g., point cloud data). Thus, the system controller 23 includes atleast one processor and/or processor circuitry for processing data, aswell as control circuitry, such as a microcontroller, that is configuredto generate control signals. The LIDAR scanning system 200 may alsoinclude a temperature sensor 26.

The receiver unit 22 includes the photodetector array 15 as well as areceiver circuit 24. The receiver circuit 24 may include one or morecircuitries or sub-circuitries for receiving and/or processinginformation. The receiver circuit 24 may receive the analog electricalsignals from the APD diodes of the photodetector array 15 and transmitthe electrical signals as raw analog data or raw digital data to thesystem controller 23. In order to transmit the raw data as digital data,the receiver circuit 24 may include an ADC and a field programmable gatearray (FPGA). The receiver circuit 24 may also receive trigger controlsignals from the system controller 23 that triggers an activation of oneor more APD diodes. The receiver circuit 24 may also receive gainsetting control signals for controlling the gain of one or more APDdiodes.

The transmitter unit 21 includes the illumination unit 10, the MEMSmirror 12, and a MEMS driver 25 configured to drive the MEMS mirror 12.In particular, the MEMS driver 25 actuates and senses the rotationposition of the mirror, and provides position information (e.g., tiltangle or degree of rotation about the rotation axis) of the mirror tothe system controller 23. Based on this position information, the lasersources of the illumination unit 10 are triggered by the systemcontroller 23 and the photodiodes (e.g., APD diodes) are activated tosense, and thus measure, a reflected light signal. Thus, a higheraccuracy in position sensing of the MEMS mirror results in a moreaccurate and precise control of other components of the LIDAR system.

The MEMS driver 25 may also measure and record mirror frequency andcurrents using a change in capacitance in a comb-drive rotor and statorof an actuator structure used to drive the MEMS mirror 12. The MEMSmirror 12 further includes the suspension structure discussed above.Thus, the MEMS driver 25 may further include a measurement circuitconfigured to measure one or more characteristics of the MEMS mirror 12described herein. The MEMS driver 25 may further include processingcircuitry, including at least one processor (e.g., analog signalprocessing circuitry and/or digital signal processing circuitry)configured to process measurement information from the measurementcircuit to evaluate a mechanical health of the MEMS mirror 12 and/or thestate of the chip package.

Additionally, or alternatively, the system controller 23 may receivemeasurement information from the measurement circuit of the MEMS driver25 and perform processing thereon. Thus, system controller 23 mayfurther include processing circuitry, including at least one processor(e.g., analog signal processing circuitry and/or digital signalprocessing circuitry) configured to process measurement information fromthe measurement circuit to evaluate a mechanical health of the MEMSmirror 12 and/or the state of the chip package.

For the QS MEMS the rotation position is measured (and controlled)continuously using the sensing circuit (at the rotor).

For a resonant MEMS: By sensing the rotation position of the MEMS mirror12 about its rotation axis (scanning axis), the MEMS driver 25 can sensezero-crossing events of the MEMS mirror 12. A zero-crossing event is aninstance when the MEMS mirror 12 has a rotation angle of 0° about itsrotation axis (scanning axis). Specifically, it is the moment when theMEMS mirror 12 is parallel to the frame or in a neutral position. Theneutral position may also be referred to as a resting position (e.g.,when the MEMS mirror 12 comes to a stop after turning off the drivingforce). Since the MEMS mirror 12 oscillates back and forth between tworotation directions (e.g., clock-wise and counter-clockwise), azero-crossing event occurs twice during a scanning period—once as themirror oscillates in the first rotation direction and once as the mirroroscillates in the second rotation direction. It will also be appreciatedthat angle-crossing events at another predefined angle may also be usedinstead of a zero-crossing event.

In some embodiments, an event time may correspond to a non-zero-crossingevent. For example, the sensed rotation angle may be some angle otherthan 0°. However, for the purpose of explanation, examples herein willbe described in the context of sensing zero-crossing events.

The MEMS driver 25 is configured to detect each zero-crossing event andrecord a timing for each event. This timing information (i.e., measuredzero-crossing time) can then be transmitted to the system controller 23as position information. Specifically, the MEMS driver 25 triggers achange in the output of a position signal (position_L) at eachzero-crossing event or angle-crossing event.

FIG. 3 illustrates a signal diagram of various signals generated by aMEMS driver 25 based on the mirror angle θ and/or position, including aposition signal (position_L). For example, the position signal(position_L) may be a pulsed signal during which a first pulsetransition (e.g., falling-edge transition) is triggered at azero-crossing as the mirror oscillates in a first rotation direction(e.g., from left to right) and a second pulse transition (e.g.,rising-edge transition) is triggered at a zero-crossing as the mirroroscillates in a second rotation direction (e.g., from right to left).Furthermore, the signal is “high” when the mirror points in onedirection (e.g., points left) and the signal is “low” when the mirrorpoints in a second direction (e.g., points right). Thus, the positionsignal not only indicates a zero-crossing event by triggering a pulsetransition, but also indicates absolute phase information by indicatingthe directional tilt of the mirror. When the interval betweenzero-crossing events increases, then the frequency of the positionsignal decreases. Based on this position signal both a phase and/or afrequency of two or more position signals can be compared.

Alternatively, a short pulse may be generated by the MEMS driver 25 ateach zero-crossing event such that a pulsed position signal (position_L)is output to the system controller 23. That is, the signal remains low(or high) between zero-crossing pulses. In this case, the absolute phaseinformation indicating which direction the mirror is moving would beabsent because the low-level (high-level) of the position signal(position_L) does not indicate whether the mirror is pointing left orright. Based on this position signal a phase and/or a frequency of twoor more position signals can be compared.

The MEMS driver 25 may send the position information to the systemcontroller 23 so that the system controller 23 can use the positioninformation to control the triggering of the laser pulses of theillumination unit 10 and the activation of the photodiodes of thephotodetector array 15. The position information may also be used by thesystem controller as feedback information such that the systemcontroller 23 can maintain a stable operation of the MEMS mirror 12 viacontrol signals provided to the MEMS driver 25 and also maintainsynchronization with other MEMS mirrors.

The MEMS mirror 12 includes an actuator structure used to drive themirror. The actuator structure includes interdigitated finger electrodesmade of interdigitated mirror combs and frame combs to which a drivevoltage v_(drive) (i.e., an actuation or driving signal) is applied bythe MEMS driver 25. The drive voltage may be referred to as ahigh-voltage (HV). The frame comb fingers and the mirror comb fingersform the electrodes of a capacitor. The drive voltage across the fingerstructure creates a driving force between interdigitated mirror combelectrodes and the frame comb electrodes, which creates a torque on themirror body 8 about the rotation axis. The drive voltage can be switchedor toggled on and off resulting in an oscillating driving force. Thedriving waveform of the drive voltage can be any waveform, includingsinusoidal, triangular, rectangular, etc. The oscillating driving forcecauses the mirror to oscillate back and forth around its rotation axisbetween two extrema. Depending on the configuration, this actuation canbe regulated or adjusted by adjusting the drive voltage off time, avoltage level of the drive voltage, or a duty cycle.

In other embodiments, an electromagnetic actuator may be used to drivethe MEMS mirror 12. For an electromagnetic actuator, a driving current(i.e., an actuation or driving signal) may be used to generate theoscillating driving force. Thus, it will be appreciated thatdrive/driving voltage and drive/driving current may be usedinterchangeably herein to indicate an actuation signal or a drivingsignal, and both may generally be referred to as a driving force.

As the mirror oscillates, the capacitance or charge between the fingerelectrodes changes according to the mirror's rotation position. The MEMSdriver 25 is configured to measure the capacitance or charge between theinterdigitated finger electrodes via, for example, the four quadrantcomb-drive displacement currents, and determine a rotation position orangle position of the MEMS mirror 12 therefrom. More specifically, thedisplacement currents are the time derivatives of the respectivecapacitances multiplied by the voltage, i.e. derivative quantities aremeasured instead of the capacitance or charge. By monitoring thedisplacement currents, the MEMS driver 25 can detect the zero-crossingevents and other non-zero angle events and timings thereof, and candetermine the deflection or the tilt angle of the MEMS mirror 12 at anygiven moment. The MEMS driver 25 can also use the measured displacementcurrents to determine a mirror frequency, and record the information ina memory at the MEMS driver 25 or at the system controller 23.

The sensing of the position of the MEMS mirror 12 is performed based ona detector that is configured to measure the capacitance by thedisplacement current or displacement charge (i.e. integrated current).The capacitance or charge sensing can be performed indirectly bymeasuring derived quantities, such as the displacement currents. Forexample, as the MEMS mirror moves, the geometry of the finger structurechanges, resulting in a change in the geometry of the capacitor. As thegeometry of the capacitor changes, the capacitance of the capacitorchanges. Thus, a specific capacitance corresponds directly to a specificdeflection position (e.g., tilt angle) of the MEMS mirror. By sensingthe capacitance of the finger structure, the MEMS driver 25 can monitorand track the oscillations of the mirror, and determine a specificposition of the MEMS mirror, including the zero-crossing.

One way to measure the capacitance is to measure a current flowingthrough the finger electrode structure, convert the measured currentinto a voltage, and then further correlate the voltage to a capacitanceand/or a rotation angle θ. However, any method to measure thecapacitance may be used.

The sign of the currents (i.e., positive or negative capacitance changeover time) only indicate if the MEMS mirror is moving towards the restposition (positive currents capacitor charging) or is moving away fromthe rest position (negative currents capacitor discharging). If the MEMSmirror is moving towards the rest position (approaching zero) this canbe both in a clockwise swing or in a counter-clockwise swing. There isno way to distinguish the rotation direction from the sign of thecurrents for comb-drives with symmetric capacitance dependence,CA(−θ_(mirror))=CA(θ_(mirror)) and CB(−θ_(mirror))=CB(θ_(mirror)).

Since the mirror is driven at an oscillation frequency (e.g., 2 kHz),when the mirror rotates in a first rotation direction (e.g.,left-to-right or clockwise), it crosses a zero position (i.e., 0°) at acertain point of time. The same can be said when the mirror rotates in asecond rotation direction (e.g., right-to-left or counter-clockwise),the mirror will cross the zero position at a certain point in time.These instances of crossing the zero position may be referred to aszero-crossing events which occur at zero-crossing times.

As noted above, one or more embodiments uses the change in a sensingsignal (e.g., a current signal) of a subset of the comb-driveelectrodes, caused by an excited parasitic mode to detect the scanningdirection (e.g., clockwise or counter-clockwise) as well as the movementamplitude and phase of parasitic modes, i.e., Ty, Tz, and Rz. Here, thedifference in the capacitance of each comb-drive, the difference in eachof the four quadrant comb-drive displacement currents, or the differenceof the displacement currents of specific combinations of the quadrantsmay be used. After detection, any of the parasitic modes Ty, Rz, Ry,and/or Tz can be avoided or actively damped.

FIG. 4A illustrates a top view of a MEMS mirror arranged in a nominalcentered position (left) and a translational shifted position (right)according to one or more embodiments. On the left, MEMS mirror 12(mirror body 8) operates in a pure rotational mode Rx about the scanningaxis 13 and the torsion bars 18 are not bending. The four comb-drives40TL, 40BL, 40TR, and 40BR are symmetrically situated within the frame17. In contrast, on the right, the MEMS mirror 12 (mirror body 8) isshifted to the right in the translational Y direction and oscillatesback and forth in the Y direction because of mode coupling of the Rxmode to the Ty mode. Here, the torsion bars 18 bend back and forth inthe Y direction. A stator electrode of a comb-drive is mechanicallycoupled to the frame 17, whereas the rotor electrode of a comb-drive ismechanically coupled to the mirror body 8. The stator comb-driveelectrodes are separated into left pairs (stators of comb-drives 40TLand 40BL) and right pairs (stators of comb-drives 40TR and 40BR) todetect the Rx-Ty-mode coupling.

Embodiments further include a sensing circuit that is electricallycoupled to the stator comb-drive electrodes of each comb-drive 40TL,40BL, 40TR, and 40BR to receive their respective displacement currentstherefrom. The stator comb-drive electrodes are electrically coupled tothe sensing circuit in such a way to detect the mode of interest such asthe Ty mode, the Rz mode, or the Tz mode. It is also possible toseparate the rotor comb-drive electrodes and to sense those electrodesinstead of the ones of the stator. Furthermore, it is not necessary tomeasure the displacement current or to use all comb-drives for thesensing. Any signal which is related to a capacitance or capacitancechange may be used.

FIG. 4B is a schematic diagram of a translational mode Ty measurementand mode damping system 400A according to one or more embodiments. Thetranslational mode Ty measurement system 400A includes the MEMS driver25, the MEMS mirror 12 with four comb-drives 40TL, 40BL, 40TR, and 40BR,a sensing circuit 50, and the system controller 23.

The MEMS driver 25 receives a digital control signal DHVact and drivesthe Rx mode of the MEMS mirror 12 (i.e., the rotation about the scanningaxis 13) in accordance with the digital control signal D_(HVact). Thedigital control signal D_(HVact) may be a single bit digital signal(i.e., indicating 0 for off and a 1 for on), or may be a multi-bitdigital signal that can be indicated different levels (e.g., differentvoltage levels). The MEMS driver 25 may include a digital to analogconverter (DAC) that generates the drive voltage v_(drive) based on thedigital value of the digital control signal DHVact. As explained below,the MEMS driver 25 may also add or superimpose a value of a dampingsignal Vdmp onto the value of the digital control signal DHVact togenerate a drive voltage with a damping voltage, or just a dampingvoltage (e.g., if the drive voltage is zero). In this way, DHVact is adigital input to a DAC that generates voltages in the range of 0V-200V.The driving voltage Vdrive can then be any waveform.

The MEMS driver 25 generates the drive voltage v_(drive) with theappropriate duty cycle and applies the drive voltage v_(drive) to themirror comb electrodes of the four comb-drives 40TL, 40BL, 40TR, and40BR. As a result of the movement of the MEMS mirror 12 about itsscanning axis 13, the drive capacitances C_(TL), C_(BL), C_(TR), andC_(BR) of the four comb-drives 40TL, 40BL, 40TR, and 40BR change anddisplacement currents i_(TL), i_(BL), i_(TR), and i_(BR) are generatedby their respective drive capacitances. The sensing circuit 50 iscoupled to the stator comb electrodes of the four comb-drives 40TL,40BL, 40TR, and 40BR and extracts the displacement currents i_(TL),i_(BL), i_(TR), and i_(BR) therefrom.

In order to measure the translational mode Ty, the two left sidedisplacement currents i_(TL) and i_(BL) are provided to an input of atransimpedance amplifier (TIA) 51 of the sensing circuit 50. While a TIAis used in this example, it will be appreciated that other sensingelements used for capacitive sensing may be used. For example, anysensing circuit that performs charge sensing, current sensing, orfrequency modulation-based sensing may be used to generate sensingsignals. TIA 51 may be coupled to the MEMS mirror 12, and specificallyto the stators of assigned comb-drives by a switch SW1. The switch SW1may be controlled by the system controller 23 (via a switch controlsignal SW CTRL) to perform mode measurements when the switch SW1 is inposition ‘m’ or to perform a damping operation when the switch SW1 is inposition ‘d’, where ‘m’ represents a measurement position of the switchfor taking measurements of the corresponding comb-drives and ‘d’represents a damping position of the switch used for applying a dampingvoltage Vdmp,left to the corresponding left-side comb-drives. As aresult of TIA 51 receiving the displacement currents in and i_(BL), thesum of the two left side displacement currents i_(TL) and i_(BL) isconverted into a left side voltage V_(L) by TIA 51.

Similarly, the two right side displacement currents i_(TR) and i_(BR)are provided to an input of a TIA 52 of the sensing circuit 50. Again,while a TIA is used in this example, it will be appreciated that othersensing elements used for capacitive sensing may be used. For example,any sensing circuit that performs charge sensing, current sensing, orfrequency modulation-based sensing may be used to generate sensingsignals. TIA 52 may be coupled to the MEMS mirror 12, and specificallyto the stators of assigned comb-drives by a switch SW2. The switch SW2may be controlled by the system controller 23 (via a switch controlsignal SW CTRL) to perform mode measurements when the switch SW2 is inposition ‘m’ or to perform a damping operation when the switch SW1 is inposition ‘d’, where ‘m’ represents a measurement position of the switchfor taking measurements of the corresponding comb-drives and ‘d’represents a damping position of the switch used for applying a dampingvoltage Vdmp,right to the corresponding right-side comb-drives. As aresult of TIA 51 receiving the displacement currents i_(TR) and i_(BR),the sum of the two right side displacement currents i_(TR) and i_(BR) isconverted into a right side voltage V_(R) by TIA 52.

Alternatively, each comb-drive electrode may be coupled to a separateTIA and then the individual voltage signals may be added and/orsubtracted such that the sensing circuit 50 is able to detect each modeat the same time. In other words, four TIAs may be used, each beingindividually connected to a respective drive electrode (i.e.,capacitances C_(TL), C_(BL), C_(TR), and C_(BR)) of the four comb-drives40TL, 40BL, 40TR, and 40BR.

FIG. 4C is a schematic diagram of a parasitic mode measurement and modedamping system 400B according to one or more embodiments. The parasiticmode measurement system 400B is similar to system 400A, with theexception that the sensing circuit 50 includes a switching network 55that switchably couples the drive electrode (i.e., capacitances C_(TL),C_(BL), C_(TR), and C_(BR)) of the four comb-drives 40TL, 40BL, 40TR,and 40BR to TIAs 51 and 52 in any combination. The switching network 55may be a network of eight switches—two for each drive electrode—thatswitchably connect their respective drive electrode to the TIAs 51 and52. Such a switching network allows the input configuration to the TIAs51 and 52 to be entirely configurable for receiving displacementcurrents i_(TL), i_(BL), i_(TR), and i_(BR) in any combination to allowthe sensing circuit 50 to measure individual parasitic modes or todetect each mode at the same time (e.g., if individual TIAs are alsoprovided). The switch control signal SW CTRL, representative of one ormore control signals, may control the switches of the switching network55. It is also possible that a network of multiplexers could be used.

As an example, two pairs of switches are shown for two drive electrodescorrespond to comb-drives 40TL and 40BR where one switch of each pair iscoupled to TIA 51 and the other one switch of each pair is coupled toTIA 52. Other switch pairs are provided, though not illustrated, in asimilar manner for the remaining comb-drives so that their respectivedisplacement currents and be switchably provided to either TIA 51 or TIA52.

Turning back to FIG. 4B, the sensing circuit 50 further includes asummer 53 and a subtractor 54. The summer 53 receives both the left sidevoltage V_(L) and the right side voltage V_(R) and sums them together togenerate a summed voltage V₁ that represents the sum of all thedisplacement currents in, i_(BL), i_(TR), and i_(BR). Conversely, thesubtractor 54 receives both the left side voltage V_(L) and the rightside voltage V_(R) and subtracts the right side voltage V_(R) from theleft side voltage V_(L) to generate a difference voltage ΔV₁ thatrepresents the difference between the left side currents i_(TL), i_(BL)and the right side currents i_(TR), i_(BR). The summed voltage V₁ (i.e.,the summed current signal or measurement signal) allows precise phase aswell as amplitude measurements of the Rx mode that the system controller23 uses for proper MEMS mirror control. However, the summed voltage V₁does not provide any information about the scanning direction, as thefour comb-drives are symmetric. On the other hand, the differencevoltage ΔV₁ (i.e., the difference current signal or differentialmeasurement signal) provides scanning direction dependent signals forthe Rx mode. It also provides the amplitude of the Ty mode and the phaseof the Ty mode, relative to the Rx mode. Thus, the system controller 23can determine the scanning direction (clockwise or counter-clockwise) ofthe MEMS mirror 12 based on the difference voltage ΔV₁. The systemcontroller 23 may generate the digital control signal D_(HVact) based onthe phase, the amplitude (θ₀), and the scanning direction. For example,the sign of the difference voltage ΔV₁ may indicate the scanningdirection.

In implementation, an additional memory can be used by the systemcontroller 23 to record the scanning direction even though thedifference voltage ΔV₁ is weak at certain operating points. The memoryor filter can be used to confirm the direction based on a weak signal orreduce the noise in the measurements, e.g. via averaging.

Another usage of the proposed method is to detect the Ty mode amplitudeby evaluating the difference voltage ΔV₁ (i.e., the difference currentsignal) by the system controller 23. This can be used for safety issuessuch as material failure or the possibility of electrostatic pull-in.

In case that the Rx trajectory is not a single sine wave, but consistsof multiple harmonics, one harmonic may hit the Ty mode resonancefrequency. Experiments showed, that for the MEMS mirror 12, the 5thharmonic of the Rx trajectory can lead to a Ty mode resonance whensweeping through the MEMS mirror response curve. However, depending onthe design of the MEMS mirror 12, the separation factors between the Rxand Ty modes may be different such that a different harmonic of the Rxtrajectory can lead to the Ty mode resonance.

If the coupling Rx-Ty-coupling via the Euler force fulfills such aresonance condition, the Ty mode is excited to much higher amplitudesthan in the non-resonant coupling case. The rotational motion of themirror can be distorted by a strong 5th (or other) harmonic, due to thelarge movement amplitude of the Ty mode. This is a consequence of the Tymode not being a pure translational mode. Rather, it contains a smallrotational component, which leads to the distortion of the overallrotation, which is the overall effect of the Rx rotation and the smallrotational contribution of the Ty mode.

The fifth (5^(th)) harmonic content of the overall rotational motionobtained by a sweep through part of the MEMS mirror response curve fordifferent driving voltages shows that the Ty response curve has an upperbranch (top response curve) with a high Ty mode amplitude as well as alower branch (bottom response curve) with low Ty mode amplitude.

The system controller 23 can be configured to monitor the differencevoltage ΔV₁ at different operating points (e.g., at the Ty moderesonance peak and at off-resonance). Based on the signal shape of thedifference voltage ΔV₁, both operation points can be clearlydistinguished. Hence, the system controller 23 is able to determinewhether the current operation point of the MEMS mirror is on the upperbranch or on the lower branch of the 5th harmonic of the response curveof the overall rotation, without the need of a trajectory measurement.Based on this information, the system controller 23 can adjust thedigital control signal D_(HVact) in order to force a jump from the upperbranch to the lower branch of the 5th harmonic. This can be achieved byan increase in the driving frequency of the digital control signalD_(HVact) beyond the fall-back frequency of the upper response curve ofthe Ty mode resonance such that the Ty mode excitation collapses. Also,the proposed mode damping can be also applied via damping signal Vdmp toforce a jump to the lower branch.

In other words, the system controller 23 can determine the Ty modeamplitude from the difference voltage ΔV₁ and determine whether the MEMSmirror 12 is on the upper branch or on the lower branch of the 5thharmonic based on the determined Ty mode amplitude. If the Ty modeamplitude exceeds a predetermined amplitude threshold, the systemcontroller 23 determines that the MEMS mirror 12 is on the upper branchof the 5th harmonic of the Rx mode and takes countermeasure by adjustingthe digital control signal D_(HVact).

After an unwanted parasitic oscillation has been detected (e.g., modeTy, mode Rz, mode Ry, or mode Tz), it can be actively damped. Because asmall Ty-mode amplitude is typically present during normal operation, athreshold should be defined for a stronger unwanted excitation of the Tymode caused by resonant coupling. That is, active damping is appliedwhen a non-zero threshold is exceeded with the threshold being set toallow for some small Ty-mode amplitude that occurs normally. Thethreshold can be determined for a specific design by experiments andnumerical simulations that off-resonance amplitudes of the Ty mode of upto 1-3 μm can occur. On-resonance coupling via a harmonic of the Rxtrajectory or by direct excitation of the comb-drive can lead to higheramplitudes. Hence, a threshold can be chosen based on a dynamic model ofthe coupling determining that amplitudes of larger than 3 μm of the Tyamplitude are considered to be caused by on-resonance coupling. Thisthreshold for the mechanical amplitude of the Ty mode can be convertedinto a corresponding threshold for the magnitude of the differencevoltage ΔV₁.

Alternatively, the frequency content of the difference voltage ΔV₁ canbe monitored by the system controller 23. Off-resonance coupling willlead to a Ty-movement at the same frequency as Rx. In contrast,on-resonance coupling, e.g. via the fifth harmonic content of the mirrortrajectory for a specific design, will lead to a sharp increase of thefifth harmonic content in the difference voltage ΔV₁. It can bedetermined that if the spectral component of the difference voltage ΔV₁corresponding to the fifth harmonic increases by a pre-determinedfactor, e.g. by a factor of 5, this is considered a detection ofon-resonance coupling. This can be done during normal operation of theMEMS mirror 12 via the system controller 23.

During an observation phase, while the driving voltage v_(drive) is on(e.g. corresponding to an on-time during which D_(HVact)=1 and v_(drive)is set to a value between 0-200V), the type of parasitic oscillation,its frequency, and its phase are determined by the system controller 23from the corresponding difference voltage ΔV₁. Thus, the systemcontroller 23 evaluates the difference voltage ΔV₁ and detects thetranslational Ty mode based on pre-determined thresholds of themagnitude of the difference voltage ΔV₁ and of the relative strength ofthe harmonic contents of the difference voltage ΔV₁.

During the off-phase of the driving voltage v_(drive) (e.g.,corresponding to an off-time during which D_(HVact)=0 and v_(drive) is0V) the TIAs 51 and 52 are disconnected from the stator electrodes(e.g., via an opened switches SW1 and SW2), and a damping voltage Vdmpis applied to the stator electrodes to counteract parasitictranslational movement Ty.

In particular, FIG. 4D illustrates a top view of a MEMS mirror beingcompensated in a first damping method (method 1) with a damping voltagein response to detecting a translational movement Ty according to one ormore embodiments. In order to damp and reduce the translational movementTy, the damping voltage Vdmp is applied to the two stator electrodeslocated on the same side of the mirror body 8 (i.e., on the same lateralside of the scanning axis 13). Thus, the damping voltage Vdmp can beapplied to either both stator electrodes of comb-drives 40TL and 40BL orto both stator electrodes of comb-drives 40TR and 40BR. The other pairof stator electrodes on the opposite side of the mirror body 8 (i.e., onthe opposite lateral side of the scanning axis 13) are grounded to 0V.As a result, the detected Ty mode is damped by applying damping voltageVdmp to counteract against the translational movement.

Alternatively, in a second damping method (method 2), the TIAs 51 and 52may remain connected to the stator electrodes (e.g., switches SW1 andSW2 are closed or are not present) during damping, and the dampingvoltage Vdmp is superimposed onto the drive voltage Vdrive. Thus, thedotted lines of the damping voltage Vdmp illustrate both options how toapply the damping voltage Vdmp depending on which damping method isused. In the alternative to applying the damping voltage Vdmp directlyto the signal line of drive voltage Vdrive, the system controller 23 maytransmit a damping control signal Ddmp to the MEMS driver 25, and theMEMS driver 25 may generate the damping voltage Vdmp with it beingsuperimposed on the drive voltage Vdrive.

In principal, both damping methods work for any mode and it depends onthe specific capacitance dependence of the overall capacitance of allfour stator quadrants on the respective degree of freedom (DOF). Due tosymmetry, the overall capacitance will always form a local extremum forany degree of freedom. For the Rx and Tz modes, the local extremum is amaximum while for Rz the local extremum is a minimum.

For the Ty mode, the situation is more complicated. In a puretranslation, the overall capacitance as a function of the DOF Ty isessentially constant because the capacitance increase on one side isexactly balanced by the capacitance decrease on the other side. This isa consequence of the fact that the finger of rotor and stator arealready engaged in the rest position (for a true in-plane drive as in agyro one would design the tips of both rotor and stator to end at thesame position, then the situation is different). If the rotationalcomponent of the Ty mode is added, the capacitance for the Ty mode has alocal maximum as well, but the dependence is still rather weak.

The force (for translational motions Ty and Tz) and the moment (forrotational motions Rz) than can be exerted by the drive with respect toa specific DOF depends on the strength of the capacitance dependence ofthe capacitor configuration that is used to force that DOF. Morespecifically, the force or torque at position q, where q is theconsidered DOF, can be expressed by F(q) or τ(q)=½∂c/∂qV². Here, Cincludes stator combs, which are used for the damping operation. In thefirst damping method, C includes the stators, to which Vdmp is applied.Since C(q) is essentially linear for the Ty case in the first dampingmethod, the derivative is essentially constant, leading to a constantfactor multiplying the applied damping voltage squared. In the seconddamping method, C includes all four stator quadrants, which results in aparabolic dependence of C around q=0. Consequently, the force or torquehas a linear dependence on q by virtue of the derivative, where theslope of the linear dependence is proportional to the curvature of thelocal extremum, which the capacitance forms at q=0. Driving or dampingwith the second damping method is in perfect analogy to the parametricexcitation of the Rx mode in normal operation.

Because the curvature of the local maximum for Ty is very low, method 1may be preferred for Ty because it leads to a much higher force usingthe linear capacitance dependence of only the left or only the rightpair of stators in an alternating fashion.

For the other DOFs, e.g. Tz, Rz, Ry, method 2 can be chosen because eachquadrant alone already forms an essentially parabolic capacitancedependence. Method 2 thus doubles the driving force/torque. It also doesnot require switches and different biasing, but the damping voltage canbe just superimposed onto the regular driving signal Vdrive.

Furthermore, method 2 may be preferred in general since the dampingsignal for the parasitic motion can be continuously applied duringon-times and off-times of the drive voltage (and not only during theoff-times). Hence, the damping signal only contains higher harmonics ofthe drive signal and does not influence the Rx motion averaged over oneRx period. In contrast, if the damping signal is only applied duringoff-times of the drive signal, it does in fact decelerate the Rx motionby virtue of the capacitance dependence on angle θ, dC/dθ, of the statorpair employed for the damping operation, where θ is the mirror angle anddC/dθ is the capacitance derivative regarding the mirror angle. For thisreason, damping method 1 has an impact on the Rx mode amplitude. FIG. 4Eshows a timing diagram for implementing the first damping method (method1) to damp parasitic mode Ty according to one or more embodiments. Here,the translational movement of the MEMS mirror 12 for the Ty mode isshown as Ymirror, whereas θmirror is the mirror angle about the scanningaxis 13 (i.e., the Rx mode). Two damping signals (voltages) Vdmp,leftand Vdmp,right are applied to left stator electrodes or to right statorelectrodes, respectively, to apply the damping during off times of thedrive voltage Vdrive to damp the Ty oscillation.

FIG. 4F shows a timing diagram for implementing the second dampingmethod (method 2) to damp parasitic mode Tz according to one or moreembodiments. Here, the translational movement of the MEMS mirror 12 forthe Tz mode is shown as Zmirror, whereas θmirror is the mirror angleabout the scanning axis 13 (i.e., the Rx mode). A damping signal(voltage) Vdmp is superimposed onto the drive voltage Vdrive during bothon times and off times based on the detection of the Tz mode to damp theTz oscillation. Considering nonlinear actuation force or torque, Vdmpcan be adjusted locally during on time and off time of Vdrive, e.g. alower amplitude of Vdmp during the on time of Vdrive.

During the observation phase, the system controller 23 determines fromthe sign of the difference voltage ΔV₁ the phase relation of the Ty-modemotion relative to the observation phase, which is the actuation phaseof the Rx-mode. This means it determines in which time intervals of theobservation phase the mirror body 8 is moving from left to right and inwhich time intervals it is moving from right to left in thetranslational Y direction. Because in the on-resonance coupling case,the frequency of the Ty-mode motion is generally at an integer multipleof the Rx-mode motion (e.g., the Ty-mode could be resonantly excited bythe fifth harmonic content of the Rx trajectory) the observation phasecan encompass both phases where the mirror body 8 is moving from left toright and phases where the mirror body 8 is moving from right to left.

During the off-phase, the damping voltage Vdmp (Vdmp,left andVdmp,right) must be alternatingly applied to either the pair formed bythe two left stator electrodes of 40TL and 40BL or to the pair formed bythe two right stator electrodes 40TR and 40BR, while the respectiveother pair may be set to 0V or some other smaller absolute value thanthe other comb drive side. This alternating signal must be constructedwith a timing or phase relation that the damping voltage Vdmp is appliedto the left pair in time periods when the Ty movement is from left toright and to the right pair in time periods when the Ty movement is fromright to left. For this signal construction, the system controller 23uses the phase information about the Ty-mode derived during theobservation phase.

A further aspect of mode coupling for a comb-drive actuated MEMS mirrorsis the coupling of the rotational mode Rx to the yaw mode Rz. Inparticular, a coupling can occur from the desired rotational mode Rx toan unwanted yaw mode Rz. The yaw mode Rz is vulnerable to directparametric excitation by the comb-drives. Direct parametric excitationis possible because of the very strong capacitance dependence for thisdegree of freedom when higher harmonics of the rectangular drive signalVdrive for the Rx mode fulfill the criterion for parametric resonance ofthe Rz mode. More specifically, if an odd integer multiple of the drivefrequency equals two times the eigenfrequency of the Rz mode, parametricexcitation of the Rz mode can occur. However, parametric resonance needsa threshold excitation strength to occur. For this reason, parametricresonance of the Rz mode does not occur if the odd integer, i.e. theorder of the higher harmonic that fulfills the resonance condition, istoo high. The exact threshold depends on the curvature of thecapacitance dependence for the Rz mode and the applied voltage. As soonas the yaw mode Rz is excited, i.e. when there is initial Rz motion,inertial coupling terms according to Euler's rotation equation lead to acoupling of the Rz and Rx mode. The yaw mode Rz can be detected andmeasured by subtracting diagonal displacement current signals.

FIG. 5A illustrates a top view of a MEMS mirror arranged in a nominalcentered position (left) and a yaw shifted position (right) according toone or more embodiments. On the left, MEMS mirror 12 (mirror body 8)operates in a pure rotational mode Rx about the scanning axis 13 and thetorsion bars 18 are not bending. The four comb-drives 40TL, 40BL, 40TR,and 40BR are symmetrically situated within the frame 17. In contrast, onthe right, the MEMS mirror 12 (mirror body 8) is shifted about the Zdirection (i.e., rotated about a Z axis) and oscillates back and forthabout the Z axis via mode coupling to the Rz mode. Here, the torsionbars 18 bend back and forth in the Y direction. The stator comb-driveelectrodes are separated into diagonal pairs of a first diagonal acrossthe MEMS chip (stators of comb-drives 40TL and 40BR) and into diagonalpairs of a second diagonal across the MEMS chip (stators of comb-drives40BL and 40TR) to detect the RxRz-mode coupling.

Embodiments further include a sensing circuit that is coupled to thestator comb-drive electrodes of each comb-drive 40TL, 40BL, 40TR, and40BR to receive their respective displacement currents therefrom. Thestator comb-drive electrodes are coupled to the sensing circuit in sucha way to detect the mode of interest such as the Ty mode or the Rz mode.

FIG. 5B is a schematic diagram of a yaw mode Rz measurement andcompensation system 500 according to one or more embodiments. The yawmode Rz measurement and compensation system 500 includes the MEMS driver25, the MEMS mirror 12 with four comb-drives 40TL, 40BL, 40TR, and 40BR,a sensing circuit 60, and the system controller 23. It will be furtherappreciated that the sensing circuits 50 and 60 can be combined to senseboth the Ty mode and the Rz mode.

The MEMS driver 25 receives a digital control signal D_(HVact) anddrives the MEMS mirror 12 in accordance with the digital control signalD_(HVact). The MEMS driver 25 generates the drive voltage v_(drive) withthe appropriate duty cycle and applies the drive voltage v_(drive) tothe mirror comb electrodes of the four comb-drives 40TL, 40BL, 40TR, and40BR. As a result of the movement of the MEMS mirror 12 about itsscanning axis 13, the drive capacitances C_(TL), C_(BL), C_(TR), andC_(BR) of the four comb-drives 40TL, 40BL, 40TR, and 40BR change anddisplacement currents i_(TL), i_(BL), i_(TR), and i_(BR) are generatedby their respective drive capacitances. The sensing circuit 60 iscoupled to the stator comb electrodes of the four comb-drives 40TL,40BL, 40TR, and 40BR and extracts the displacement currents i_(TL),i_(BL), i_(TR), and i_(BR) therefrom.

In order to measure the yaw mode Rz, the two diagonal or catty-cornerdisplacement currents in and i_(BR) are provided to an input of a TIA 61of the sensing circuit 60. While a TIA is used in this example, it willbe appreciated that other sensing elements used for capacitive sensingmay be used. For example, any sensing circuit that performs chargesensing, current sensing, or frequency modulation-based sensing may beused to generate sensing signals. TIA 61 may be coupled to the MEMSmirror 12, and specifically to the stators of assigned comb-drives. Thediagonal displacement currents in and i_(BR) are extracted from statorcomb electrodes of the comb-drives 40TL and 40BR that are diagonallyarranged across the mirror body 8 from each other. As a result, the sumof the two diagonal displacement currents in and i_(BR) is convertedinto a first diagonal voltage VD1 by TIA 61.

Similarly, the two other diagonal or catty-corner displacement currentsi_(TR) and i_(BL) are provided to an input of a TIA 62 of the sensingcircuit 60. Again, while a TIA is used in this example, it will beappreciated that other sensing elements used for capacitive sensing maybe used. For example, any sensing circuit that performs charge sensing,current sensing, or frequency modulation-based sensing may be used togenerate sensing signals. TIA 62 may be coupled to the MEMS mirror 12,and specifically to the stators of assigned comb-drives. The diagonaldisplacement currents in and i_(BL) are extracted from stator combelectrodes of the comb-drives 40TR and 40BL that are diagonally arrangedacross the mirror body 8 from each other. As a result, the sum of thetwo other diagonal displacement currents in and i_(BL) is converted intoa second diagonal voltage VD2 by TIA 62.

The sensing circuit 60 further includes a summer 63 and a subtractor 64.The summer 63 receives both the first diagonal voltage VD1 and thesecond diagonal voltage VD2 and sums them together to generate a summedvoltage V₁ that represents the sum of all the displacement currentsi_(TL), i_(BL), i_(TR), and i_(BR). Conversely, the subtractor 64receives both first diagonal voltage VD1 and the second diagonal voltageVD2 and subtracts the second diagonal voltage VD2 from the firstdiagonal voltage VD1 to generate a difference voltage ΔV₁ thatrepresents the difference between the first pair of diagonal currentsin, i_(BR) (i.e., a sum thereof) and the second pair of diagonalcurrents in, i_(BL) (i.e., a sum thereof). The summed voltage V₁ (i.e.,the summed current signal) allows precise phase as well as amplitudemeasurements of the Rx mode that the system controller 23 uses forproper MEMS mirror control. However, the summed voltage V₁ does notprovide any information about the scanning direction, as the fourcomb-drives are symmetric. On the other hand, the difference voltage ΔV₁(i.e., the difference current signal) provides information correspondingto the amplitude of the yaw mode Rz and the phase of the yaw mode Rz,relative to the Rx mode. For example, the difference voltage ΔV₁ mayoscillate between positive and negative values as the MEMS mirror 12oscillates about the Z axis. The system controller 23 may generate thedigital control signal D_(HVact) based on the phase and the amplitude ofthe Rx mode (rotation angle θ_(mirror)).

Additionally, after an unwanted parasitic oscillation has been detected(e.g., mode Ty, mode Rz, mode Ry, or mode Tz), it can be actively dampedusing method 2 described above, which superimposes a damping signal Vdmponto the drive signal Vdrive.

A threshold in the difference voltage ΔV₁ can be defined to determinethat the yaw mode Rz is excited. The threshold of the difference voltageΔV₁ can be determined such that it is equivalent to an Rz rotationangle, which is considered critical. Since in the Rz mode, the combfingers of the rotor and stators approach each other due to aconsiderable transversal displacement component, which can lead to thepotentially destructive effect of electrostatic pull-in, the thresholdangle for critical Rz motion is usually chosen very low. In someembodiments, it can be chosen to be in the range of 0.015° to 0.15°. Asuitable threshold angle can be determined for a specific design byexperiments and numerical simulations and converted into a correspondingthreshold for the magnitude of the difference voltage ΔV₁ consideringthe readout electronics. Additionally, the spectral components of thedifference voltage ΔV₁ can be analyzed to determine if an enhancedcomponent at the expected frequency of the yaw mode Rz is present. Thethreshold criterion described above can be applied to only the spectralcomponent at the expected frequency of the yaw mode Rz in order to makethe detection more robust by excluding other spectral components, whichcan indicate small acceptable non-resonant motion in the yaw mode Rz,e.g. caused by asymmetries due to fabrication. These procedures can beapplied during normal operation of the MEMS mirror 12 via the systemcontroller 23.

During an initial observation phase, while the driving voltage v_(drive)is on (e.g. corresponding to an on-time during which D_(HVact)=1 andv_(drive) is set to a predetermined voltage, e.g., 0-200V), the type ofparasitic oscillation, its amplitude, its frequency, and its phaserelative to the operational mode Rx are determined by the systemcontroller 23 from the corresponding difference voltage ΔV₁. Thus, thesystem controller 23 evaluates the difference voltage ΔV₁ and detectsthe rotational yaw mode Rz based on pre-determined thresholds of themagnitude of the difference voltage ΔV₁ or of the magnitude of thespectral component of the difference voltage ΔV₁ at the expectedfrequency of the yaw mode Rz.

After an initial observation phase, a damping voltage Vdmp issuperimposed onto the drive voltage v_(drive). This means it is appliedto the rotor electrodes, while the stator electrodes remain grounded.The damping voltage Vdmp is at twice the frequency of the parasitic yawmode and its phase relative to v_(drive) is chosen based on thedetermination of the phase of the yaw mode Rz relative to theoperational mode Rx such that the Rz mode motion is decelerated. Forinstance, both the driving voltage v_(drive) and the damping voltageVdmp can be unipolar square waves and the frequency of the dampingvoltage Vdmp is at an integer multiple of the frequency of the drivingvoltage v_(drive) in the case of a resonant parasitic excitation. Incontrast to the method 1 described for the damping of the parasitic Tymode, the damping voltage for the Rz mode is not only applied during theoff-phase of the driving voltage, but also during the on-phase. This ispossible since it can be performed with the same biasing of rotor andstators, respectively, as the application of the drive voltage v_(drive)Furthermore, no switches are required.

In particular, FIG. 5C illustrates a top view of a MEMS mirror beingcompensated with a damping voltage in response to detecting a yawmovement Rz according to one or more embodiments. Here, a dampingvoltage Vdmp is superimposed onto the drive voltage v_(drive) that isapplied to the rotor. The stator electrodes of 40TL, 40TR, 40BL, and40BR are grounded. Vdmp can be locally adjusted during on time and offtime of v_(drive) considering nonlinear actuation force or torque.

In order to damp and reduce the yaw movement Rz, the damping voltageVdmp is applied to the rotor electrode by superimposition onto thedriving voltage v_(drive) while the stator electrodes remain grounded to0V for monitoring the displacement currents. As a result, the detectedRz mode is damped by applying damping voltage Vdmp by superimpositiononto driving voltage v_(drive) in order to counteract against theunwanted rotational movement.

In subsequent observation phases, the amplitude and the phase of the yawmode Rz can be continuously monitored and the damping voltage Vdmp canbe adjusted in amplitude and phase to the updated information about theamplitude and the phase of the Rz mode.

During the observation phase, the system controller 23 determines fromthe difference voltage ΔV₁ the phase of the Rz motion. Morespecifically, it determines whether the mirror body 8 is approaching therest position or moving away from the rest position related to the Rzdegree of freedom. If the mirror is approaching the Rz-related restposition, it reduces the Rz-related capacitance, leading to dischargingcurrents. If the mirror is moving away from the Rz-related restposition, it increases the Rz-related capacitance, leading to chargingcurrents. The charging and discharging currents lead to respectivedifference voltages with opposite signs. By this procedure, the phase ofthe Rz-motion can be determined relative to the drive signal. Inresponse to this phase detection of the movement about the Z axis, thedamping voltage Vdmp can subsequently be applied by superimposition ontothe driving signal v_(drive) during the phases when the mirror isapproaching the Rz-related rest (middle) position, while no additionalvoltage is superimposed during the phases when the mirror is moving awayfrom the Rz-related rest position. The Rz-related capacitance has aminimum at the Rz-related rest position. For this reason, application ofa voltage Vdmp between rotor and stators pulls the mirror away from theRz-related rest position. If voltage is applied during the phases, inwhich the mirror approaches the Rz-related rest position, theRz-movement is therefore decelerated, i.e. damped. The Rz-motion iscontinuously monitored and the application of the damping voltage Vdmpis adjusted based on updated phase and amplitude information about themotion of the yaw mode Rz.

Furthermore, if the stator comb-drive electrodes of each of the fourcomb-drives 40TL, 40BL, 40TR, and 40BR are separated into two layers ofequal thickness, a translational mode Tz in the Z direction (i.e.,out-of-plane upward and downward translational movement) can be detectedby the difference of front-side and back-side layer currents, where“front-side” relates to the layer, which is at a larger (i.e. morepositive) z-coordinate with respect to the coordinate system of FIG. 1BConversely, “back-side” denotes the layer, which is at a lower (i.e.more negative) z-coordinate with respect to the coordinate system ofFIG. 1B.

FIGS. 6A and 6B illustrate cross-sectional views of a MEMS mirror anddriver electrodes in accordance with one or more embodiments. Inparticular, in FIG. 6A, the left stator comb-drive electrode 43 includestwo layers, a front-side layer 43 a and a back-side layer 43 b, of equalthickness. Similarly, the right stator comb-drive electrode 44 includestwo layers, a front-side layer 44 a and a back-side layer 44 b, of equalthickness. In contrast, in FIG. 6B, the respective front-side andback-side layers have different thicknesses.

A mode coupling between the Rx mode and the Tz mode can be caused by adisplacement L between the center of mass M from the rotational axis(scanning axis 13), due to the reinforcement structure 9. This inertialcoupling originates from the sum of a Euler force component and acentrifugal component according to Equation 2:

f _(Z)(t)=−m Lθ _(m)(t){umlaut over (θ)}_(m)(t)−mL({dot over(θ)}_(m)(t))  Eq. 2,

with the mirror mass m, the mirror angle θ_(m)(t), the mirror angularvelocity {dot over ( )} θ_(m)(t) and the mirror rotational acceleration{umlaut over (θ)}_(m)(t). Thus, the Tz mode is excited by the inertialforce f_(Z)(t) having a frequency content at twice the harmonicfrequencies of the Rx trajectory θ(t) by virtue of the dependence on themirror angle and its time derivatives. If f_(Z)(t) features a frequencycontent at the resonance frequency of the Tz mode, resonant excitationof the Tz mode can occur. A different coupling mechanism between the Rxmode and the Tz mode is direct parametric excitation of the Tz-mode byhigher harmonic content of the driving voltage v_(drive) of the Rx-mode.

The subtraction of all back-side layer displacement currents from alldisplacement currents of the front-side layer allows the detection ofthe Tz movement via a difference signal. The detection of the Tzmovement is also possible for not equally thick layers, but the obtaineddifference current needs further processing by the system controller 23because the desired Rx mode also causes a nonzero signal. Thus,non-equal thicknesses causes a difference in front-side and back-sidecurrents for both the Rx and the Tz modes.

FIG. 6C is a schematic diagram of a Tz mode measurement and compensationsystem 600 according to one or more embodiments. The Tz mode measurementand compensation system 600 includes the MEMS driver 25, the MEMS mirror12 with four comb-drives 40TL, 40BL, 40TR, and 40BR, each consisting ofa fronts-side layer and a back-side layer, a sensing circuit 70, and thesystem controller 23. It will be further appreciated that the sensingcircuits 50, 60 and 70 can be combined to sense both the Ty mode, the Rzmode and the Tz mode.

The MEMS driver 25 receives a digital control signal D_(HVact) anddrives the MEMS mirror 12 in accordance with the digital control signalD_(HVact). The MEMS driver 25 generates the drive voltage vane with theappropriate duty cycle and applies the drive voltage vane to both layersof the mirror comb electrodes of the four comb-drives 40TL, 40BL, 40TR,and 40BR. As a result of the movement of the MEMS mirror 12 about itsscanning axis 13, the drive capacitances C_(TL-FS), C_(BL-FS),C_(TR-FS), C_(BR-FS), C_(TL-BS), C_(BL-BS), C_(TR-BS) and C_(BR-BS) ofthe four comb-drives 40TL, 40BL, 40TR, and 40BR change and displacementcurrents i_(TL-FS), i_(BL_FS), i_(TR_FS), i_(BR-FS), i_(TL-BS),i_(BL_BS), i_(TR_BS) and i_(BR-BS) are generated by their respectivedrive capacitances. The sensing circuit 70 is coupled to the stator combelectrodes of the four comb-drives 40TL, 40BL, 40TR, and 40BR andextracts the displacement currents i_(TL-FS), i_(BL_FS), i_(TR_FS),i_(BR-FS), i_(TL-BS), i_(BL_BS), i_(TR_BS) and i_(BR-BS) therefrom.

In order to measure the Tz mode, all front-side displacement currentsi_(TL-FS), i_(BL_FS), i_(TR_FS) and i_(BR-FS) are provided to an inputof a TIA 71 of the sensing circuit 70. TIA 71 may be coupled to the MEMSmirror 12, and specifically to the front-side layers of the stators ofall comb-drives. The front-side displacement currents i_(TL-FS),i_(BL_FS), i_(TR_FS) and i_(BR-FS) are extracted from the front-sidelayers of the stator comb electrodes of the comb-drives 40TL, 40BL,40TR, and 40BR. As a result, the sum of all front-side displacementcurrents i_(TL-FS), i_(BL_FS), i_(TR_FS) and i_(BR-FS) is converted intoa first voltage V_(FS) by TIA 71.

Similarly, all back-side displacement currents i_(TL-BS), i_(BL_BS),i_(TR_BS) and i_(BR-BS) are provided to an input of a TIA 72 of thesensing circuit 70. TIA 72 may be coupled to the MEMS mirror 12, andspecifically to the back-side layers of the stators of all comb-drives.The back-side displacement currents i_(TL-BS), i_(BL_BS), i_(TR_BS) andi_(BR-BS) are extracted from the back-side layers of the stator combelectrodes of the comb-drives 40TL, 40BL, 40TR, and 40BR. As a result,the sum of all back-side displacement currents i_(TL-BS), i_(BL_BS),i_(TR_BS) and i_(BR-BS) is converted into a second voltage V_(BS) by TIA72.

The sensing circuit 70 further includes a summer 73 and a subtractor 74.The summer 73 receives both the first voltage V_(FS) and the secondvoltage V_(BS) and sums them together to generate a summed voltage V₁that represents the sum of all the displacement currents i_(TL-FS),i_(BL_FS), i_(TR_FS), i_(BR-FS), i_(TL-BS), i_(BL_BS), i_(TR_BS) andi_(BR-BS). Conversely, the subtractor 74 receives both first voltageV_(FS) and the second voltage V_(BS) and subtracts the second voltageV_(BS) from the first voltage V_(FS) to generate a difference voltageΔV₁ that represents the difference between all front-side currentsi_(TL-FS), i_(BL) FS, i_(TR_FS) and i_(BR-FS) and all back-side currentsi_(TL-BS), i_(BL_BS), i_(TR_BS) and i_(BR-BS). The summed voltage V₁(i.e., the summed current signal) allows precise phase as well asamplitude measurements of the Rx mode that the system controller 23 usesfor proper MEMS mirror control. The difference voltage ΔV₁ (i.e., thedifference current signal) provides information corresponding to theamplitude of the Tz mode and the phase of the Tz mode relative to the Rxmode. For example, the difference voltage ΔV₁ may oscillate betweenpositive and negative values as the MEMS mirror 12 oscillates along theZ direction. The system controller 23 may generate the digital controlsignal D_(HVact) based on the phase and the amplitude of the Rx mode(rotation angle θ_(mirror)).

Additionally, after an unwanted parasitic oscillation of the Tz mode hasbeen detected, it can be actively damped. A threshold in the differencevoltage ΔV₁ can be defined to determine that the Tz mode is excited. Thethreshold of the difference voltage can be determined such that it isequivalent to a Tz displacement, which is considered critical. Since inthe Tz mode, additional mechanical stress is exerted onto the mirrorsuspensions, particularly the torsion bars, the threshold displacementfor critical Tz motion is usually chosen to be low. In embodiments, itcan be chosen to be in the range of 1.5-10 μm. A suitable thresholddisplacement can be determined for a specific design by experiments andnumerical simulations and converted into a corresponding threshold forthe magnitude of the difference voltage ΔV₁ considering the readoutelectronics. Additionally, the spectral components of the differencevoltage ΔV₁ can be analyzed to determine if an enhanced component at theexpected frequency of the Tz mode exists. The threshold criteriondescribed above can be applied to only the spectral component at theexpected frequency of the Tz mode in order to make the detection morerobust by excluding other spectral components, which can indicate smallacceptable non-resonant motion in the Tz mode, e.g. caused byasymmetries due to fabrication. These procedures can be applied duringnormal operation of the MEMS mirror 12 via the system controller 23.

During an initial observation phase, while the driving voltage v_(drive)is on (e.g. corresponding to an on-time during which D_(HVact)=1 andv_(drive) is set to a predetermined voltage, e.g., 0-200V), the type ofparasitic oscillation, its frequency, and its phase relative to theoperational mode Rx are determined by the system controller 23 from thecorresponding difference voltage ΔV₁. Thus, the system controller 23evaluates the difference voltage ΔV₁ and detects the translational Tzmode based on pre-determined thresholds of the magnitude of thedifference voltage ΔV₁ or of the magnitude of the spectral component ofthe difference voltage ΔV₁ at the expected frequency of thetranslational Tz mode.

After an initial observation phase, a damping voltage Vdmp issuperimposed onto the drive voltage v_(drive). This means it is appliedto the rotor, while the stator electrodes remain grounded. The dampingvoltage Vdmp is at twice the frequency of the parasitic translational Tzmode and its phase relative to v_(drive). is chosen based on thedetermination of the phase of the translational Tz mode relative to theoperational mode Rx such that the Tz mode motion is decelerated. Forinstance, both the driving voltage v_(drive) and the damping voltageVdmp can be unipolar square waves and the frequency of the dampingvoltage Vdmp is at an integer multiple of the frequency of the drivingvoltage v_(drive) in the case of a resonant parasitic excitation. Indifference to the method described for the damping of the parasitic Tymode, the damping voltage for the Tz mode is not only applied during theoff-phase of the driving voltage v_(drive), but also during theon-phase. This is possible since it can be performed with the samebiasing of rotor and stators, respectively, as the application of thedrive voltage v_(drive). Furthermore, no switches are required.

In particular, FIG. 6D illustrates a cross-sectional view of a MEMSmirror being compensated with a damping voltage in response to detectinga Tz movement according to one or more embodiments. In order to damp andreduce the Tz movement, the damping voltage Vdmp is applied to the rotorelectrode by superimposition onto the driving voltage v_(drive) whilethe stator electrodes remain grounded to 0V for monitoring thedisplacement currents. As a result, the detected Tz mode is damped byapplying damping voltage Vdmp by superimposition onto driving voltagev_(drive) in order to counteract against the unwanted translationalmovement.

In subsequent observation phases, the amplitude and the phase of the Tzmode can be continuously monitored and the damping voltage Vdmp can beadjusted in amplitude and phase according to the updated informationabout the amplitude and the phase of the Tz mode.

During the observation phase, the system controller 23 determines fromthe difference voltage ΔV₁ the phase of the Tz motion. Morespecifically, it determines whether the mirror body 8 is approaching therest position or moving away from the rest position related to the Tzdegree of freedom. If the mirror is approaching the Tz-related restposition, it increases the Tz-related capacitance, leading to currentscharging the comb capacitors. If it is approaching from negative Z-side,mainly the capacitors related to the front-side layers are charged,leading to nonzero currents i_(TL-FS), i_(BL-FS), i_(TR-FS) andi_(BR-FS). If it is approaching from positive Z-side, mainly thecapacitors related to the back-side layers are charged, leading tononzero currents i_(TL-BS), i_(BL-BS), i_(TR-BS) and i_(BR-BS) If themirror is moving away from the Tz-related rest position, it decreasesthe Tz-related capacitance, leading to currents discharging the combcapacitors. If it is distancing on the negative Z-side, mainly thecapacitors related to the front-side layers are discharged, leading tononzero currents i_(TL-FS), i_(BL-FS), i_(TR-FS) and i_(BR-FS) with theopposite sign compared to the approaching case. If it is distancing onthe positive Z-side, mainly the capacitors related to the back-sidelayers are discharged, leading to nonzero currents i_(TL-BS), i_(BL-BS),i_(TR-Bs) and i_(BR-BS) with the opposite sign compared to theapproaching case. These signals lead to associated difference voltages.The loading and unloading of back- or frontside currents also depends onthe current rotation angle.

By a suitable analysis of the difference voltage, the phase of theTz-motion can be established relative to the driving signal. In responseto this phase detection of the movement along the Z axis, the dampingvoltage Vdmp can subsequently be applied by superimposition onto thedriving signal v_(drive) during the phases when the mirror is movingaway from the Tz-related rest (middle) position, while no additionalvoltage is superimposed during the phases when the mirror is approachingthe Tz-related rest position. The Tz-related capacitance has a maximumat the Tz-related rest position. For this reason, application of avoltage Vdmp between rotor and stators pulls the mirror towards theTz-related rest position. If voltage is applied during the phases, inwhich the mirror is moving away from the Tz-related rest position, theTz-movement is therefore decelerated, i.e. damped. The Tz-motion iscontinuously monitored and the application of the damping voltage Vdmpis adjusted based on updated phase and amplitude information about themotion of the translational Tz mode.

The Tz mode coupling can be detected by monitoring the difference signalof the front-side layer of the stators and the back-side layers of thestator. The frequency location of the mode coupling to the Tz mode canbe recorded and saved in the memory to avoid the future excitation ofthe Tz mode, for example in a new start-up of the mirror when thefrequency of the driving voltage is swept in open loop to start the Rxoscillations for acquisition of initial signals. A large change of themirror position in the translational Z direction can be used by thesystem controller 23 for the detection of a malfunction of the mirror12. For example, if the difference signal exceeds a predeterminedamplitude threshold.

FIG. 6E depicts four capacitor currents DL1R, DL2R, DL1L, and DL2L in anexcitation frequency sweep over a wide range from 4 kHz to 45 kHz for aMEMS mirror according one or more embodiments. The capacitor currentsDL1R, DL2R, DL1L, and DL2L are related to FIGS. 6B and 6C. DL1R meansfor example the sum of front-side layer (DL1 . . . Device Layer 1=frontside layer) currents of the right side. DL2L means the sum of backsidelayer currents of the left side, etc. The capacitor currents DL1R, DL2R,DL1L, and DL2L can be generalized as displacement currents ixL-FS=DL1L,ixR-FS=DL1R, ixL-BS=DL2L, ixR-BS=DL2R where “bottom” and “top” layercurrents are not discriminated here (summed by jointly contacting) suchthat “x” can apply to a bottom or top layer current. Direct excitationsof the operational mode Rx and of the parasitic modes Tz and Rz at twicetheir respective resonance frequency are observed. This means they areexcited in first order parametric resonance. However, mode Rz is alsoexcited at a third of its parametric resonance frequency. This can beexplained by the square wave driving via v_(drive) which has higher, oddnumbered harmonics at 3, 5, 7, . . . times the fundamental frequency.Thus, the third harmonic content of the square wave driving fulfills theresonance criterion for first order parametric resonance of yaw mode Rz.By the excitation frequency sweep, this behavior is provoked. However,such unwanted excitations can also occur in normal operation, i.e. whenthe driving frequency is at twice the Rx frequency, if a parasitic modefulfills the criterion Rx·(2n+1)=Rp, where Rp is the frequency of theparasitic mode and n=0, 1, 2, . . . Such parasitic excitations can bedetected and identified by the methods described before.

After an unwanted parasitic oscillation has been detected (e.g., modeTy, mode Rz, mode Ry, or mode Tz), it can be actively damped. This canbe done during normal operation of the MEMS mirror 12 via the systemcontroller 23.

During an observation phase, while the driving voltage v_(drive) is on(e.g. while D_(HVact)=1 and v_(drive) is set to a value between 0-200V),the type of parasitic oscillation, its frequency, and its phase aredetermined by the system controller 23 from the corresponding differencevoltage ΔV₁.

In particular, the Tz mode can be actively damped in response to itsdetection. When the rotor moves from the center upwards or downwards, avoltage Vdamp is applied between rotor and stator electrodes. When therotor moves back towards the center, no voltage (0V) is applied.

FIG. 7A illustrates a top view of a quasi-static (QS) MEMS mirrorarranged in a nominal centered position (left) and a translationalshifted position (right) according to one or more embodiments. For theQS MEMS mirror 12 the stator combs are used for driving the MEMS mirror12 about the rotation axis 13 and the rotor is used for sensing via asensing circuit. For QS mirrors the stator comb electrodes (typicallyeither the left or the right, where the other is set to zero voltage)are applied with two individual drive voltages, while the rotor combelectrodes are assigned to ground.

FIG. 7B is a schematic diagram of a Ty mode measurement and dampingsystem 700 for a QS MEMS mirror according to one or more embodiments.The Ty mode measurement system 700 includes a MEMS driver 25 thatincludes two high voltage (HV) drivers 25 a and 25 b that each apply anindividual drive voltage to a respective set of stator comb electrodes.In particular, HV driver 25 a applies a drive voltage to the stator combelectrodes of comb-drives 40TL and 40BL (i.e., the left side) and HVdriver 25 b applies a driving voltage to the stator comb electrodes ofcomb-drives 40TR and 40BR (i.e., the right side). For QS mirrors the twoindividual drive voltages are applied such, that the QS MEMS mirror 12tilts either in positive or negative direction. Applying both drivevoltages at the same time is usually avoided to not excite the Tz mode.Each HV driver 25 a and 25 b receives a respective digital controlsignal D_(HVleft) or digital control signal D_(HVright) and drives theMEMS mirror 12 in accordance with its respective digital control signalin a similar manner described above when using digital control signalD_(HVact).

For QS mirror sensing of the Rx nominal mode and the Ty parasitic mode,a sensing circuit 80 (e.g., a TIA) is connected to the rotor combs tomeasure the capacitance by the displacement charge or the displacementcurrent isens received from the rotor (i.e., the rotor comb electrodes).

For QS mirror sensing, a high frequency modulation voltage can be addedto the nominal driving voltage to allow a mirror position sensing (Rx)and parasitic mode sensing (Ty) independent from the applied drivingvoltage.

For QS mirror sensing, the Ty parasitic mode is distinguished from theRx nominal mode by its frequency component by the system controller 23.Once the system controller 23 detects the Ty mode and determines itsvalue is above certain predetermined threshold, the system controller 23can actively damp the Ty mode by adding a counter voltage Vdmp to thedriving comb electrodes on one side or the other, or in alternativefashion as the driving voltages are applied, respectively. That is, acounter voltage Vdmp is superimposed onto a driving voltage as thedriving voltage is being applied to the right side stator combelectrodes to the left side stator comb electrodes. Summers 81 and 82may be used to add or combine the counter voltage (damping voltage) Vdmponto the digital control signals D_(HVleft) and D_(HVright). so that thecounter voltage Vdmp is superimposed onto the driving voltage Vdrive bythe respective HV driver 25 a, 25 b. The stator comb electrodes of theopposite side are grounded when not being driven/damped. Vdmp can bescaled by Vdrive considering nonlinear actuation force and torque.

FIG. 7C illustrates a top view of a QS MEMS mirror being compensatedwith a damping voltage in response to detecting a translational movementTy according to one or more embodiments. The damping voltage beingapplied to counter the Ty mode is applied to the left side stator combelectrodes and the right side stator comb electrodes in an alternatingmanner to be superimposed onto the driving voltage Vdrive as it isalternatively applied according to the digital control signalsD_(HVleft) and D_(HVright). Ground alternates with Vdrive/Vdmp as shown.

FIG. 7D illustrates a cross-sectional view of a QS MEMS mirror beingcompensated with a damping voltage in response to detecting atranslational movement Ty according to one or more embodiments. Here,when the stator comb-drive electrodes 43 and 44 have two layers(front-side top layer 43 a, 44 a and bottom back-side layer 43 b, 44 b),the driving voltage Vdrive and the damping voltage Vdmp can be applied,alternating with ground, side-to-side. Here, the stator electrode (44 ain FIG. 7D top and 43 a FIG. 7D bottom) may be applied with the dampingvoltage Vdmp, such that nominal Rx drive and Ty compensation are appliedseparated from each other.

FIG. 7E illustrates a top view of a QS MEMS mirror being compensatedwith an alternative method using dedicated sensing combs to detect atranslational movement Ty according to one or more embodiments. It willbe appreciated that a resonant MEMS mirror may also use dedicatedsensing combs. Thus, dedicated sensing combs are not limited to QS MEMSmirrors, but can be applied to any MEMS mirror, including thosedescribed herein.

Here, the QS MEMS mirror 12 includes four dedicated sensing combs 45TL,45BL, 45TR, and 45BR, each with a stator comb electrode fixed to themirror frame 17 and used for measuring the capacitance by thedisplacement charge or the displacement current received from the statorcomb electrodes, as similarly described above with respect to, forexample, FIGS. 4B and 4C. The dedicated sensing combs 45TL, 45BL, 45TR,and 45BR also have a respective rotor comb electrode that is grounded.

Once the system controller 23 detects the Ty mode and determines itsvalue is above certain predetermined threshold, the system controller 23can actively damp the Ty mode by adding a counter voltage Vdmp to thedriving stator comb electrodes when a corresponding driving voltageVdrive is applied to that electrode. The application of Vdrive/Vdmpalternates side-to-side based on the digital control signals D_(HVleft)and D_(HVright).

It is also noted that the sensing combs 45TL, 45BL, 45TR, and 45BR canbe electrically coupled to a sensing circuit (e.g., TIAs, adders andsubtractors) in a similar manner shown in 4B, 4C, 5B, 6C, and 7B (e.g.,as done for drive-cobs 40TL, 405BL, 40TR, and 40BR), not only fordetecting and identifying one or more parasitic modes, but also fordetermining the phase, amplitude, and scanning direction of therotational Rx mode. Additionally, the type of parasitic oscillation(i.e., whether it is a Ty, Rz, Ry, or Tz mode), its frequency, and itsphase can be determined in a similar manner described above, forexample, by analyzing a differential measurement signal. Thus, thesensing combs are used for sensing and generating measurement signals,and the drive-combs are used for driving the MEMS mirror 12 and fordamping identified parasitic modes.

After detection, not only can any of the parasitic modes Ty, Rz, Ry,and/or Tz can be actively damped, but they can be preemptively avoided.Parasitic modes of Ty and Tz each have a high Q factor of its parasiticdynamics of Rx mode. The high Q factor causes large coupling to the Rxmode, which can lead to inaccuracy of the scan angle by scantrajectories distortion, and malfunction of the mirror control system.In worst case, a parasitic mode can destroy the MEMS mirror by largedisplacement of the comb-drives and fatigues in a weak structure of theMEMS mirror. However due to the innate high Q factor, large couplingoccurs at a very small frequency region and also varies by theoperational condition, such as peak input voltage, duty cycle, andexcitation waveform shape. A parasitic mode can show hysteric behaviorif it contains nonlinear spring constant, e.g. stiffening or softening.Those large parasitic mode couplings should not be used as operationalpoints for the LIDAR operation or should be avoided during the start-upoperation of the MEMS mirror 12 to avoid potential damage or fatigue.

FIG. 8 illustrates a flow diagram of a parasitic mode coupling avoidancemethod according to one or more embodiments. When a parasitic couplingmode is detected during the operation of the MEMS mirror 12 (operation805), the mode coupling influence is evaluated (operation 810), e.g. theconditions and frequency region of the parasitic mode, how intense themode coupling is, and if it is the catastrophic point, which aredetected as critically influencing or can be destructive to the MEMSmirror. In particular, the frequency location of the Rx mode coupling tothe parasitic mode can be determined, along with the amplitude of theparasitic mode, and the amplitude and frequency location (region) of theparasitic mode coupling is recorded in memory (operation 820). If theamplitude and frequency location (region) of the parasitic mode couplingare determined in operation 825 to be at an operational point of the Rxmode and are determined to be critically influencing or destructive tothe MEMS mirror, then the system controller 23 blocks or avoids thatoperational point in operation 830 by modifying the operation conditionof the MEMS mirror 12 via applying a frequency shift or duty cyclechange for driving the Rx mode. That is, if the harmonic numbers of theparasitic mode coupling is critical according to its frequency locationrelative to the operational point of the Rx mode and/or according to theamplitude of the parasitic mode coupling, that operational point ofdriving the Rx mode is avoided so as to not excite the correspondingparasitic mode.

If the system controller 23 determines that the harmonics (frequencylocation) of the parasitic mode are not located at the operational pointof the Rx mode, but only located at some of the start-up points of theMEMS mirror and potentially cause a damage to the MEMS mirror 12 basedon, for example, their amplitude exceeding a predetermined threshold(operation 835), then the system controller 23 can modify the startup ofthe MEMS mirror 12 in operation 840 to avoid such peak couplingfrequencies or its near frequencies. The system controller 23 can bealso modify operation conditions during startup, e.g., peak inputvoltages, duty cycles, and input waveform of the driving voltage Vdrivein operation 840.

When it is at the operational point for a desired operating frequencyand amplitude of mode Rx, the operational frequency and amplitude of thedriving voltage Vdrive can be shifted to the other value in operation830, e.g., shifting to a slightly higher operating frequency that haslower parasitic couplings. The amplitude can be adjusted for the targetoperating condition of Rx by the system controller 23 by adjustingeither the duty cycle or voltage scale of the driving voltage Vdrive.

If the system controller 23 does not determine a parasitic mode to be atthe operational point of the Rx mode in operation 825 or to bedestructive for the MEMS mirror in operation 835, the system controller23 continues to record the parasitic coupling behavior to the Rx mode inoperation 845 and returns to operation 825 for continuous monitoringuntil entire frequencies for a region of a parasitic mode are fullyanalyzed.

In case of duffing oscillation of the coupling mode, its hystericbehavior can be used, e.g. a slight increase of the frequency for thefallback of the coupling mode and slight down sweep to operation at thesame operation condition, which can reduce coupling effect significantlyby the hysteric behavior while keeping the operation condition asdesired. For example, the system controller 23 may evaluate thedifference in the Rx trajectory when the Ty mode is excited at highamplitude and at low amplitude. This is due to the hysteretic (duffing)behavior of the Ty mode. For example, the 5th harmonic content of the Rxtrajectory of the MEMS mirror 12 caused by a high Ty mode amplitude maybe evaluated. When sweeping the driving signal from low frequencies tohigh frequencies (starting the MEMS mirror up at run up to the desiredoperation point) an upper branch of the response curve of the MEMSmirror 12 is followed until a fallback point at high Ty amplitude to thelower branch of the response curve happens. If the high Ty modeamplitude is detected (i.e. by capacitive readout), a jump from theupper branch to the lower brunch can be initiated by the systemcontroller 23 to avoid large parasitic Ty mode coupling. A similarevaluation can be applied to the other parasitic modes.

Additional embodiments are provided below:

1. A scanning system, comprising:

a microelectromechanical system (MEMS) scanning structure configured torotate about an axis with a desired rotational mode of movement based onat least one driving signal;

a plurality of comb-drives configured to drive the MEMS scanningstructure about the axis according to the desired rotational mode ofmovement based on the at least one driving signal, wherein eachcomb-drive comprises a rotor comb electrode and a stator comb electrodethat form a capacitive element that has a capacitance that depends onthe deflection angle of the MEMS scanning structure;

a driver configured to generate the at least one driving signal;

a sensing circuit selectively coupled to at least a subset of theplurality of comb-drives for receiving sensing signals therefrom,wherein each sensing signal is representative of the capacitance of acorresponding comb-drive; and

a processing circuit configured to determine a scanning direction of theMEMS scanning structure in the desired rotational mode of movement basedon the sensing signals.

2. The scanning system of embodiment 1, wherein:

the plurality of comb-drives includes a first subset of comb-drives anda second subset of comb-drives,

the sensing circuit is configured to subtract at least one sensingsignal provided by the second subset of comb-drives from at least onesensing signal provided by the first subset of comb-drives to generate adifferential measurement signal, and

the processing circuit is configured to determine the scanning directionof the MEMS scanning structure in the desired rotational mode ofmovement based on the differential measurement signal.

3. The scanning system of embodiment 2, wherein the sign of thedifferential measurement signal indicates the scanning direction.

4. The scanning system of embodiment 2, further comprising:

a system controller configured to modify the at least one driving signalbased on the determined scanning direction.

5. The scanning system of embodiment 2, wherein:

the sensing circuit is configured to add the at least one sensing signalprovided by the first subset of comb-drives and the at least one sensingsignal provided by the second subset of comb-drives to generate a summedmeasurement signal, and

the processing circuit is configured to determine at least one of aphase or an amplitude of the desired rotational mode of movement basedon the summed measurement signal.

6. The scanning system of embodiment 5, further comprising:

a system controller configured to modify the at least one driving signalbased on at least one of the determined phase of the desired rotationalmode of movement or the determined amplitude of the desired rotationalmode of movement.

7. The scanning system of embodiment 1, wherein the processing circuitis configured to detect and identify a parasitic mode of movement of theMEMS scanning structure based on the sensing signals.

8. The scanning system of embodiment 7, wherein:

the plurality of comb-drives includes a first subset of comb-drives anda second subset of comb-drives,

the sensing circuit is configured to subtract at least one sensingsignal provided by the second subset of comb-drives from at least onesensing signal provided by the first subset of comb-drives to generate adifferential measurement signal, and

the processing circuit is configured to identify the parasitic mode ofmovement, determine an amplitude of the parasitic mode of movement, anddetermine a phase of the parasitic mode of movement relative to the atleast one driving signal based on the differential measurement signal.

9. The scanning system of embodiment 8, further comprising:

a system controller configured to dampen the identified parasitic modeof movement by applying at least one damping signal, wherein the systemcontroller adjusts an amplitude and a phase of the at least one dampingsignal based on the amplitude and the phase of the parasitic mode ofmovement.

10. The scanning system of embodiment 1, further comprising:

a system controller,

wherein the processing circuit is configured to determine an amplitudeand a phase of a parasitic mode of movement of the MEMS scanningstructure relative to the at least one driving signal based on thesensing signals, and

the system controller is configured to dampen the identified parasiticmode of movement by applying at least one damping signal, wherein thesystem controller adjusts an amplitude and a phase of the at least onedamping signal based on the amplitude and the phase of the parasiticmode of movement.

11. The scanning system of embodiment 7, further comprising:

a system controller configured to dampen the identified parasitic modeof movement based on the identified parasitic mode of movement byapplying at least one damping signal directly to at least a subset ofstator comb electrodes of the plurality of comb-drives or directly toeach rotor comb electrode of the plurality of comb-drives.

12. The scanning system of embodiment 7, further comprising:

a system controller configured to dampen the identified parasitic modeof movement based on the identified parasitic mode of movement bysuperimposing the at least one damping signal onto the at least onedriving signal.

13. The scanning system of embodiment 1, wherein:

the plurality of comb-drives includes a first subset of comb-drives anda second subset of comb-drives,

the sensing circuit is configured to add the sensing signals from thefirst subset of comb-drives to generate a first summed sensing signal,add the sensing signals from the second subset of comb-drives togenerate a second summed sensing signal, and subtract the second summedsensing signal from the first summed sensing signal to generate adifferential measurement signal, and

the processing circuit is configured to determine the scanning directionof the MEMS scanning structure in the desired rotational mode ofmovement based on the differential measurement signal.

14. The scanning system of embodiment 1, wherein:

the plurality of comb-drives includes a first subset of comb-drivesarranged laterally from the axis in a first direction and a secondsubset of comb-drives arranged laterally from the axis in a seconddirection opposite to the first direction,

the sensing circuit is configured to add the sensing signals from thefirst subset of comb-drives to generate a first summed sensing signal,add the sensing signals from the second subset of comb-drives togenerate a second summed sensing signal, and subtract the second summedsensing signal from the first summed sensing signal to generate adifferential measurement signal, and

the processing circuit is configured to determine the scanning directionof the MEMS scanning structure in the desired rotational mode ofmovement based on the differential measurement signal.

15. The scanning system of embodiment 1, wherein:

the plurality of comb-drives includes a first subset of comb-drivesarranged diagonally from each other across the axis on a first diagonaland a second subset of comb-drives arranged diagonally from each otheracross the axis on a second diagonal that intersects with the firstdiagonal,

the sensing circuit is configured to add the sensing signals from thefirst subset of comb-drives to generate a first summed sensing signal,add the sensing signals from the second subset of comb-drives togenerate a second summed sensing signal, and subtract the second summedsensing signal from the first summed sensing signal to generate adifferential measurement signal, and

the processing circuit is configured to determine the scanning directionof the MEMS scanning structure in the desired rotational mode ofmovement based on the differential measurement signal.

16. A scanning system, comprising:

a microelectromechanical system (MEMS) scanning structure configured torotate about an axis with a desired rotational mode of movement based onat least one driving signal;

a plurality of comb-drives configured to drive the MEMS scanningstructure about the axis according to the desired rotational mode ofmovement based on the at least one driving signal, wherein eachcomb-drive comprises a rotor comb electrode and a stator comb electrodethat form a capacitive element that has a capacitance that depends onthe deflection angle of the MEMS scanning structure;

a driver configured to generate the at least one driving signal;

a sensing circuit selectively coupled to at least a subset of theplurality of comb-drives for receiving sensing signals therefrom,wherein each sensing signal is representative of the capacitance of acorresponding comb-drive; and

a processing circuit configured to detect and identify a parasitic modeof movement of the MEMS scanning structure based on the sensing signals.

17. The scanning system of embodiment 16, further comprising:

a system controller configured to dampen the identified parasitic modeof movement based on the identified parasitic mode of movement byapplying at least one damping signal to at least a subset of stator combelectrodes of the plurality of comb-drives or to each rotor combelectrode of the plurality of comb-drives.

18. The scanning system of embodiment 17, wherein the system controllerselectively determines the subset of stator comb electrodes to which theat least one damping signal is applied based on the identified parasiticmode.

19. The scanning system of embodiment 17, wherein the system controlleris configured to superimpose the at least one damping signal onto the atleast one driving signal.

20. The scanning system of embodiment 16, further comprising:

a system controller configured selectively couple the sensing circuit toat least the subset of the plurality of comb-drives for the sensingcircuit to receive sensing signals therefrom based on a type of theparasitic mode of movement being evaluated.

21. The scanning system of embodiment 16, wherein:

the plurality of comb-drives includes a first subset of comb-drives anda second subset of comb-drives,

the sensing circuit is configured to subtract at least one sensingsignal provided by the second subset of comb-drives from at least onesensing signal provided by the first subset of comb-drives to generate adifferential measurement signal, and

the processing circuit is configured to identify a parasitic mode ofmovement of the MEMS scanning structure, determine an amplitude of theparasitic mode of movement, and determine a phase of the parasitic modeof movement relative to the at least one driving signal based on thedifferential measurement signal.

22. The scanning system of embodiment 21, further comprising:

a system controller configured to dampen the identified parasitic modeof movement by applying at least one damping signal, wherein the systemcontroller adjusts an amplitude and a phase of the at least one dampingsignal based on the amplitude and the phase of the parasitic mode ofmovement.

23. The scanning system of embodiment 16, further comprising:

a system controller configured to dampen the identified parasitic modeof movement by biasing a pair of stator comb electrodes during a drivevoltage off-time of the driving signal to decelerate the identifiedparasitic mode of movement in accordance to the identified parasiticmode of movement,

wherein the MEMS scanning structure is a resonant MEMS scanningstructure.

24. The scanning system of embodiment 16, further comprising:

a system controller configured to dampen the identified parasitic modeof movement by superimposing a damping voltage onto the at least onedriving signal that is applied to each of the rotor comb electrodes todecelerate the identified parasitic mode of movement,

wherein the MEMS scanning structure is a resonant MEMS scanningstructure.

25. The scanning system of embodiment 16, further comprising:

a system controller configured to dampen the identified parasitic modeof movement by superimposing a damping voltage onto the at least onedriving signal that is applied to a pair of stator comb electrodes todecelerate the identified parasitic mode of movement,

wherein the MEMS scanning structure is a quasistatic MEMS scanningstructure.

26. A scanning system, comprising:

a microelectromechanical system (MEMS) scanning structure configured torotate about an axis with a desired rotational mode of movement based onat least one driving signal;

a plurality of comb-drives configured to drive the MEMS scanningstructure about the axis according to the desired rotational mode ofmovement based on the at least one driving signal, wherein eachcomb-drive comprises a rotor drive-comb electrode and a statordrive-comb electrode;

a driver configured to generate the at least one driving signal;

a plurality of sensing combs, wherein each sensing comb comprises arotor sense-comb electrode and a stator sense-comb electrode that form acapacitive element that has a capacitance that depends on the deflectionangle of the MEMS scanning structure;

a sensing circuit selectively coupled to at least a subset of theplurality of sensing combs for receiving sensing signals therefrom,wherein each sensing signal is representative of the capacitance of acorresponding sensing combs; and

a processing circuit configured to detect and identify a parasitic modeof movement of the MEMS scanning structure based on the sensing signals.

27. The scanning system of embodiment 26, further comprising:

a system controller configured to dampen the identified parasitic modeof movement according to the identified parasitic mode of movement byapplying at least one damping signal to at least a subset of statordrive-comb electrodes of the plurality of comb-drives or to each rotordrive-comb electrode of the plurality of comb-drives.

28. The scanning system of embodiment 27, wherein the system controllerselectively determines the subset of stator drive-comb electrodes towhich the at least one damping signal is applied based on the identifiedparasitic mode.

29. The scanning system of embodiment 27, wherein the system controlleris configured to superimpose the at least one damping signal onto the atleast one driving signal.

30. The scanning system of embodiment 26, wherein the processing circuitis configured to determine a scanning direction of the MEMS scanningstructure in the desired rotational mode of movement based on thesensing signals.

31. The scanning system of embodiment 30, wherein:

the plurality of sensing combs includes a first subset of sensing combsand a second subset of sensing combs,

the sensing circuit is configured to subtract at least one sensingsignal provided by the second subset of sensing combs from at least onesensing signal provided by the first subset of sensing combs to generatea differential measurement signal, and

the processing circuit is configured to determine the scanning directionof the MEMS scanning structure in the desired rotational mode ofmovement based on the differential measurement signal

32. The scanning system of embodiment 31, wherein:

the sensing circuit is configured to add the sensing signals from thefirst subset of sensing combs and the second subset of sensing combs togenerate a summed measurement signal, and

the processing circuit is configured to determine at least one of aphase or an amplitude of the desired rotational mode of movement basedon the summed measurement signal.

33. The scanning system of embodiment 26, wherein:

the plurality of sensing combs includes a first subset of sensing combsand a second subset of sensing combs,

the sensing circuit is configured to subtract at least one sensingsignal provided by the second subset of sensing combs from at least onesensing signal provided by the first subset of sensing combs to generatea differential measurement signal, and

the processing circuit is configured to identify the parasitic mode ofmovement, determine an amplitude of the parasitic mode of movement, anddetermine a phase of the parasitic mode of movement relative to the atleast one driving signal based on the differential measurement signal.

34. The scanning system of embodiment 33, further comprising:

a system controller configured to dampen the identified parasitic modeof movement by applying at least one damping signal, wherein the systemcontroller adjusts an amplitude and a phase of the at least one dampingsignal based on the amplitude and the phase of the parasitic mode ofmovement.

35. The scanning system of embodiment 26, further comprising:

a system controller configured to dampen the identified parasitic modeof movement by superimposing a damping voltage onto the at least onedriving signal that is applied to a pair of stator drive-comb electrodesto decelerate the identified parasitic mode of movement.

36. A scanning system, comprising:

a microelectromechanical system (MEMS) scanning structure configured torotate about an axis with a desired rotational mode of movement based onat least one driving signal;

a plurality of combs-drives configured to drive the MEMS scanningstructure about the axis according to the desired rotational mode ofmovement based on the at least one driving signal, wherein eachcomb-drive comprises a rotor comb electrode and a stator comb electrodethat form a capacitive element that has a capacitance that depends onthe deflection angle of the MEMS scanning structure;

a driver configured to generate the at least one driving signal;

a system controller configured to shift a driving frequency of the atleast one driving signal to provoke a parasitic mode coupling betweenthe desired rotational mode of movement and a parasitic mode of movementof the MEMS scanning structure;

a sensing circuit selectively coupled to at least a subset of theplurality of comb-drives for receiving sensing signals therefrom,wherein each sensing signal is representative of the capacitance of acorresponding comb-drive; and

a processing circuit configured to determine a frequency range of thedriving frequency at which the parasitic mode coupling occurs,

wherein the system controller controls the at least one driving signalto avoid the parasitic mode.

37. The scanning system of embodiment 36, wherein the system controllercontrols at least one of the driving frequency, a peak input voltage, aduty cycle, or a waveform of the at least one driving signal such thatthe parasitic mode is avoided.

38. A scanning system, comprising:

a microelectromechanical system (MEMS) scanning structure configured torotate about an axis with a desired rotational mode of movement based onat least one driving signal;

a plurality of comb-drives configured to drive the MEMS scanningstructure about the axis according to the desired rotational mode ofmovement based on the at least one driving signal, wherein eachcomb-drive comprises a rotor drive-comb electrode and a statordrive-comb electrode;

a driver configured to generate the at least one driving signal;

a plurality of sensing combs, wherein each sensing comb comprises arotor sense-comb electrode and a stator sense-comb electrode that form acapacitive element that has a capacitance that depends on the deflectionangle of the MEMS scanning structure;

a system controller configured to shift a driving frequency of the atleast one driving signal to provoke a parasitic mode coupling betweenthe desired rotational mode of movement and a parasitic mode of movementof the MEMS scanning structure;

a sensing circuit selectively coupled to at least a subset of theplurality of sensing combs for receiving sensing signals therefrom,wherein each sensing signal is representative of the capacitance of acorresponding sensing comb; and

a processing circuit configured to determine a frequency range of thedriving frequency at which the parasitic mode coupling occurs,

wherein the system controller controls the at least one driving signalto avoid the parasitic mode.

39. The scanning system of embodiment 38, wherein the system controllercontrols at least one of the driving frequency, a peak input voltage, aduty cycle, or a waveform of the at least one driving signal such thatthe parasitic mode is avoided.

Although embodiments described herein relate to a MEMS device with amirror, it is to be understood that other implementations may includeoptical devices other than MEMS mirror devices, including otheroscillating structures, including those not related to LIDAR. Inaddition, although some aspects have been described in the context of anapparatus, it is clear that these aspects also represent a descriptionof the corresponding method, where a block or device corresponds to amethod step or a feature of a method step. Analogously, aspectsdescribed in the context of a method step also represent a descriptionof a corresponding block or item or feature of a correspondingapparatus. Some or all of the method steps may be executed by (or using)a hardware apparatus, like for example, a microprocessor, a programmablecomputer or an electronic circuit. In some embodiments, some one or moreof the method steps may be executed by such an apparatus.

While various embodiments have been described, it will be apparent tothose of ordinary skill in the art that many more embodiments andimplementations are possible within the scope of the disclosure.Accordingly, the invention is not to be restricted except in light ofthe attached claims and their equivalents. With regard to the variousfunctions performed by the components or structures described above(assemblies, devices, circuits, systems, etc.), the terms (including areference to a “means”) used to describe such components are intended tocorrespond, unless otherwise indicated, to any component or structurethat performs the specified function of the described component (i.e.,that is functionally equivalent), even if not structurally equivalent tothe disclosed structure that performs the function in the exemplaryimplementations of the invention illustrated herein.

Furthermore, the following claims are hereby incorporated into thedetailed description, where each claim may stand on its own as aseparate example embodiment. While each claim may stand on its own as aseparate example embodiment, it is to be noted that—although a dependentclaim may refer in the claims to a specific combination with one or moreother claims—other example embodiments may also include a combination ofthe dependent claim with the subject matter of each other dependent orindependent claim. Such combinations are proposed herein unless it isstated that a specific combination is not intended. Furthermore, it isintended to include also features of a claim to any other independentclaim even if this claim is not directly made dependent to theindependent claim.

It is further to be noted that methods disclosed in the specification orin the claims may be implemented by a device having means for performingeach of the respective acts of these methods.

Further, it is to be understood that the disclosure of multiple acts orfunctions disclosed in the specification or in the claims may not beconstrued as to be within the specific order. Therefore, the disclosureof multiple acts or functions will not limit these to a particular orderunless such acts or functions are not interchangeable for technicalreasons. Furthermore, in some embodiments a single act may include ormay be broken into multiple sub acts. Such sub acts may be included andpart of the disclosure of this single act unless explicitly excluded.

Instructions may be executed by one or more processors, such as one ormore central processing units (CPU), digital signal processors (DSPs),general purpose microprocessors, application specific integratedcircuits (ASICs), field programmable logic arrays (FPGAs), or otherequivalent integrated or discrete logic circuitry. Accordingly, the term“processor” or “processing circuitry” as used herein refers to any ofthe foregoing structure or any other structure suitable forimplementation of the techniques described herein. In addition, in someaspects, the functionality described herein may be provided withindedicated hardware and/or software modules. Also, the techniques couldbe fully implemented in one or more circuits or logic elements.

Thus, the techniques described in this disclosure may be implemented, atleast in part, in hardware, software, firmware, or any combinationthereof. For example, various aspects of the described techniques may beimplemented within one or more processors, including one or moremicroprocessors, DSPs, ASICs, or any other equivalent integrated ordiscrete logic circuitry, as well as any combinations of suchcomponents.

A controller including hardware may also perform one or more of thetechniques described in this disclosure. Such hardware, software, andfirmware may be implemented within the same device or within separatedevices to support the various techniques described in this disclosure.Software may be stored on a non-transitory computer-readable medium suchthat the non-transitory computer readable medium includes a program codeor a program algorithm stored thereon which, when executed, causes thecontroller, via a computer program, to perform the steps of a method.

Although various exemplary embodiments have been disclosed, it will beapparent to those skilled in the art that various changes andmodifications can be made which will achieve some of the advantages ofthe concepts disclosed herein without departing from the spirit andscope of the invention. It will be obvious to those reasonably skilledin the art that other components performing the same functions may besuitably substituted. It is to be understood that other embodiments maybe utilized and structural or logical changes may be made withoutdeparting from the scope of the present invention. It should bementioned that features explained with reference to a specific figuremay be combined with features of other figures, even in those notexplicitly mentioned. Such modifications to the general inventiveconcept are intended to be covered by the appended claims and theirlegal equivalents.

What is claimed is:
 1. A scanning system, comprising: amicroelectromechanical system (MEMS) scanning structure configured torotate about an axis with a desired rotational mode of movement based onat least one driving signal; a plurality of comb-drives configured todrive the MEMS scanning structure about the axis according to thedesired rotational mode of movement based on the at least one drivingsignal, wherein each comb-drive comprises a rotor comb electrode and astator comb electrode that form a capacitive element that has acapacitance that depends on the deflection angle of the MEMS scanningstructure; a driver configured to generate the at least one drivingsignal; a sensing circuit selectively coupled to at least a subset ofthe plurality of comb-drives for receiving sensing signals therefrom,wherein each sensing signal is representative of the capacitance of acorresponding comb-drive; and a processing circuit configured todetermine a scanning direction of the MEMS scanning structure in thedesired rotational mode of movement based on the sensing signals.
 2. Thescanning system of claim 1, wherein: the plurality of comb-drivesincludes a first subset of comb-drives and a second subset ofcomb-drives, the sensing circuit is configured to subtract at least onesensing signal provided by the second subset of comb-drives from atleast one sensing signal provided by the first subset of comb-drives togenerate a differential measurement signal, and the processing circuitis configured to determine the scanning direction of the MEMS scanningstructure in the desired rotational mode of movement based on thedifferential measurement signal.
 3. The scanning system of claim 2,wherein the sign of the differential measurement signal indicates thescanning direction.
 4. The scanning system of claim 2, furthercomprising: a system controller configured to modify the at least onedriving signal based on the determined scanning direction.
 5. Thescanning system of claim 2, wherein: the sensing circuit is configuredto add the at least one sensing signal provided by the first subset ofcomb-drives and the at least one sensing signal provided by the secondsubset of comb-drives to generate a summed measurement signal, and theprocessing circuit is configured to determine at least one of a phase oran amplitude of the desired rotational mode of movement based on thesummed measurement signal.
 6. The scanning system of claim 5, furthercomprising: a system controller configured to modify the at least onedriving signal based on at least one of the determined phase of thedesired rotational mode of movement or the determined amplitude of thedesired rotational mode of movement.
 7. The scanning system of claim 1,wherein the processing circuit is configured to detect and identify aparasitic mode of movement of the MEMS scanning structure based on thesensing signals.
 8. The scanning system of claim 7, wherein: theplurality of comb-drives includes a first subset of comb-drives and asecond subset of comb-drives, the sensing circuit is configured tosubtract at least one sensing signal provided by the second subset ofcomb-drives from at least one sensing signal provided by the firstsubset of comb-drives to generate a differential measurement signal, andthe processing circuit is configured to identify the parasitic mode ofmovement, determine an amplitude of the parasitic mode of movement, anddetermine a phase of the parasitic mode of movement relative to the atleast one driving signal based on the differential measurement signal.9. The scanning system of claim 8, further comprising: a systemcontroller configured to dampen the identified parasitic mode ofmovement by applying at least one damping signal, wherein the systemcontroller adjusts an amplitude and a phase of the at least one dampingsignal based on the amplitude and the phase of the parasitic mode ofmovement.
 10. The scanning system of claim 1, further comprising: asystem controller, wherein the processing circuit is configured todetermine an amplitude and a phase of a parasitic mode of movement ofthe MEMS scanning structure relative to the at least one driving signalbased on the sensing signals, and the system controller is configured todampen the identified parasitic mode of movement by applying at leastone damping signal, wherein the system controller adjusts an amplitudeand a phase of the at least one damping signal based on the amplitudeand the phase of the parasitic mode of movement.
 11. The scanning systemof claim 7, further comprising: a system controller configured to dampenthe identified parasitic mode of movement based on the identifiedparasitic mode of movement by applying at least one damping signaldirectly to at least a subset of stator comb electrodes of the pluralityof comb-drives or directly to each rotor comb electrode of the pluralityof comb-drives.
 12. The scanning system of claim 7, further comprising:a system controller configured to dampen the identified parasitic modeof movement based on the identified parasitic mode of movement bysuperimposing the at least one damping signal onto the at least onedriving signal.
 13. The scanning system of claim 1, wherein: theplurality of comb-drives includes a first subset of comb-drives and asecond subset of comb-drives, the sensing circuit is configured to addthe sensing signals from the first subset of comb-drives to generate afirst summed sensing signal, add the sensing signals from the secondsubset of comb-drives to generate a second summed sensing signal, andsubtract the second summed sensing signal from the first summed sensingsignal to generate a differential measurement signal, and the processingcircuit is configured to determine the scanning direction of the MEMSscanning structure in the desired rotational mode of movement based onthe differential measurement signal.
 14. The scanning system of claim 1,wherein: the plurality of comb-drives includes a first subset ofcomb-drives arranged laterally from the axis in a first direction and asecond subset of comb-drives arranged laterally from the axis in asecond direction opposite to the first direction, the sensing circuit isconfigured to add the sensing signals from the first subset ofcomb-drives to generate a first summed sensing signal, add the sensingsignals from the second subset of comb-drives to generate a secondsummed sensing signal, and subtract the second summed sensing signalfrom the first summed sensing signal to generate a differentialmeasurement signal, and the processing circuit is configured todetermine the scanning direction of the MEMS scanning structure in thedesired rotational mode of movement based on the differentialmeasurement signal.
 15. The scanning system of claim 1, wherein: theplurality of comb-drives includes a first subset of comb-drives arrangeddiagonally from each other across the axis on a first diagonal and asecond subset of comb-drives arranged diagonally from each other acrossthe axis on a second diagonal that intersects with the first diagonal,the sensing circuit is configured to add the sensing signals from thefirst subset of comb-drives to generate a first summed sensing signal,add the sensing signals from the second subset of comb-drives togenerate a second summed sensing signal, and subtract the second summedsensing signal from the first summed sensing signal to generate adifferential measurement signal, and the processing circuit isconfigured to determine the scanning direction of the MEMS scanningstructure in the desired rotational mode of movement based on thedifferential measurement signal.
 16. A scanning system, comprising: amicroelectromechanical system (MEMS) scanning structure configured torotate about an axis with a desired rotational mode of movement based onat least one driving signal; a plurality of comb-drives configured todrive the MEMS scanning structure about the axis according to thedesired rotational mode of movement based on the at least one drivingsignal, wherein each comb-drive comprises a rotor comb electrode and astator comb electrode that form a capacitive element that has acapacitance that depends on the deflection angle of the MEMS scanningstructure; a driver configured to generate the at least one drivingsignal; a sensing circuit selectively coupled to at least a subset ofthe plurality of comb-drives for receiving sensing signals therefrom,wherein each sensing signal is representative of the capacitance of acorresponding comb-drive; and a processing circuit configured to detectand identify a parasitic mode of movement of the MEMS scanning structurebased on the sensing signals.
 17. The scanning system of claim 16,further comprising: a system controller configured to dampen theidentified parasitic mode of movement based on the identified parasiticmode of movement by applying at least one damping signal to at least asubset of stator comb electrodes of the plurality of comb-drives or toeach rotor comb electrode of the plurality of comb-drives.
 18. Thescanning system of claim 17, wherein the system controller selectivelydetermines the subset of stator comb electrodes to which the at leastone damping signal is applied based on the identified parasitic mode.19. The scanning system of claim 17, wherein the system controller isconfigured to superimpose the at least one damping signal onto the atleast one driving signal.
 20. The scanning system of claim 16, furthercomprising: a system controller configured selectively couple thesensing circuit to at least the subset of the plurality of comb-drivesfor the sensing circuit to receive sensing signals therefrom based on atype of the parasitic mode of movement being evaluated.
 21. The scanningsystem of claim 16, wherein: the plurality of comb-drives includes afirst subset of comb-drives and a second subset of comb-drives, thesensing circuit is configured to subtract at least one sensing signalprovided by the second subset of comb-drives from at least one sensingsignal provided by the first subset of comb-drives to generate adifferential measurement signal, and the processing circuit isconfigured to identify a parasitic mode of movement of the MEMS scanningstructure, determine an amplitude of the parasitic mode of movement, anddetermine a phase of the parasitic mode of movement relative to the atleast one driving signal based on the differential measurement signal.22. The scanning system of claim 21, further comprising: a systemcontroller configured to dampen the identified parasitic mode ofmovement by applying at least one damping signal, wherein the systemcontroller adjusts an amplitude and a phase of the at least one dampingsignal based on the amplitude and the phase of the parasitic mode ofmovement.
 23. The scanning system of claim 16, further comprising: asystem controller configured to dampen the identified parasitic mode ofmovement by biasing a pair of stator comb electrodes during a drivevoltage off-time of the driving signal to decelerate the identifiedparasitic mode of movement in accordance to the identified parasiticmode of movement, wherein the MEMS scanning structure is a resonant MEMSscanning structure.
 24. The scanning system of claim 16, furthercomprising: a system controller configured to dampen the identifiedparasitic mode of movement by superimposing a damping voltage onto theat least one driving signal that is applied to each of the rotor combelectrodes to decelerate the identified parasitic mode of movement,wherein the MEMS scanning structure is a resonant MEMS scanningstructure.
 25. The scanning system of claim 16, further comprising: asystem controller configured to dampen the identified parasitic mode ofmovement by superimposing a damping voltage onto the at least onedriving signal that is applied to a pair of stator comb electrodes todecelerate the identified parasitic mode of movement, wherein the MEMSscanning structure is a quasistatic MEMS scanning structure.